Yeah, you first need to understand the basics of the metering system and the exposure modes, when and why to use them and how they effect setting up your camera.
There are two manual modes on your camera: M and B. M or Manual is used most of the time, for exposures up to 30 seconds. B or Bulb is used for longer exposures, upwards of 30 seconds.
Neither of those manual modes rely upon the meter built into the camera. It can be used to determine exposure, though, if you wish. The scale shown in the viewfinder or the camera's LCD screen is a readout for the meter. Essentially, if you "center" the pointer on that scale, you are using the camera's recommended settings (i.e., it's the same as the camera would use in one of the auto modes). In M or B mode you also can vary the setting as you see fit, and/or use a separate meter to determine what ISO, aperture and shutter speed combination to use. It is possible to get pretty good at setting exposure by eye, then perhaps checking the results on the camera's histogram display and tweaking things a little.
The other exposure modes all have more or less automation. Tv and Av are Shutter Preferred and Aperture Preferred, respectively. In the first case, after setting your ISO you choose the shutter speed you want the camera to use and it will automatically choose the aperture it thinks will provide correct exposure. In the latter case, you chose the aperture and the camera automatically chooses what it thinks is an appropriate shutter speed.
P or Program is a mode where you set the ISO and the camera chooses both the aperture and the shutter speed.
All three of these are fully dependent upon the metering system in the camera. That system has three or four metering modes and some nuances that you need to understand and learn to correct for in certain circumstances. The metering modes are Evaluative, Center Weighted, Partial and Spot. Not all camera models have a Spot mode. Evaluative is a fairly sophisticated mode that puts extra emphasis on the area right around the active AF point, which presumably is on the main subject, but also averages out the exposure over the whole image area. This works pretty well in many situations. If the subject is filling enough of the frame area, it can even handle backlit situations pretty well. Center Weighted is sort of an old fashioned mode, where the most emphasis is put on the center, but the entire image area is averaged. Partial metering measures a more defined, central area of the image and Spot takes it even farther, measuring just the area shown in a circle within the viewfinder and largely ignoring the rest of the image area.
Why would you choose Av over Tv or P? Well, if what's important making a particular image is depth of field, then Av would be the choice, so that you can select the apeture you want the camera to use. If stopping movement (or deliberately blurring it) is what's most important, then Tv would be a good choice. Personally I don't use P a lot, because one or the other of the above considerations is important to me... But I do use it sometimes when I'm shooting fast, don't have time to stop and think about my settings, and am going from one lighting situation to another very different one quickly (such as from shooting inside to taking a couple quick outdoor shots, then returning to the indoor shooting).
Now, one of the problems with a camera's metering system is that it's reflective... That means it senses and measures the light being reflected off of the subject. Because of this, it's directly effected by the subject's tonality. For example, under exactly the same lighting, you'll get quite different metering readings pointing the camera toward a groom dressed in a black tux vs pointing it toward a bride dressed in a white gown. In other words, the camera has no way of knowing the subject's tonality... So the best it can do is to try to render everything a "medium gray". Left to do this, the groom in the black tux ends up over exposed and the bride in the white gown will beunder exposed... Unless we make some adjustments. If you happen to be taking a shot with both the bride's and groom's clothing equally within the metered area, the camera should average to a correct exposure. But if the image is dominated by or the area being metered by the camera is particularly light colored or a particularly dark tonality subject, some adjustments have to be made.
To correct for the "errors" inherent with reflective metering systems, Exposure Compensation is a feature provided as a means of overriding the settings the camera is trying to make. For the black subject you would want to dial in some minus exposure. For the white subject you'll want to dial in some plus exposure. Just how much is something you learn to judge by doing it... with practice.
Exposure Compensation or EC uses the same scale in Canon cameras, as is used for a meter readout in M mode. Just remember, in any of the auto exposure modes that's an EC scaled... Only in M mode is it a meter readout.
ETTR or "Expose To The Right" is simply a practice of setting the camera up in any of the auto exposure modes with Exposure Compensation to be slightly biased toward plus exposure, all the time. It's just trying to apply a standardized setting on all situations, because it's a little safer to slightly over expose a shot than it is to accidentally under expose. That's because under exposure leads to more noise, when you later adjust the image by increasing exposure in post processing. If you don't dial in any + EC and just go out and shoot, then note later that you are having to increase exposure a lot in post processing, it might be a good idea to set up an leave some + EC on your camera.
A bit of ETTR works best when shooting RAW files, because there is an abundance of data recorded in the lighter areas of images and it is usually no problem to dial down an exposure by a 1/3 to 2/3 stop later in post processing. RAW files contain a lot more of the original data than JPEGs straight from the camera. So while it's possible to use ETTR when shooting JPEGs only, too, it's safer doing so with RAW files than JPEGs.
Usually people using ETTR only dial in + 1/3 to +2/3 EC. So it's not a lot.
Ideally, though, you wouldn't use ETTR at all but instead would fine tune your exposures before each and every shot, carefully considering the subject's tonalities and making any necessary adjustments. However, out in the real world and not having time to do adjustments like that all the time, ETTR is often an easier solution.
There is no EC in M mode, so no ETTR either. If you are in M mode and using the scale, you still can dial in some + or - if you wish, but it's not ETTR. It's simply biasing your manual settings a little toward over or under exposure. Ultimately, though, the reason for doing so and the results are the same as using ETTR in an auto exposure mode.
There are other methods of metering that give more accuracy, too. An incidence type meter measures the light falling onto a subject, rather than what's being reflected off of it. So an incidence meter is not "fooled" by differences in subject tonality. There's no practical way to build an incidence meter into a camera, so it means using a separate handheld meter, setting the camera to M and making your own settings. Again, it's slower to do this, but if time allows and lighting is steady enough that you don't need to vary settings a lot during a shoot, it might be helpful to get one of these meters and learn to use it well.
But, again, you can learn to set exposure by eye, too. For example, there is the "Sunny 16 Rule" which says that on a sunny day you can set the aperture to f16, the ISO to whatever you want, and the shutter speed you'll need will be the reciprocal of the ISO. So, for example, if you set ISO 100, then your shutter speed would be 1/100. You can vary this, too... Say if you want to use f8 instead, that's two stops larger aperture so you need to shorten your shutter speed by two stops to compensate, in this case to 1/400. You can learn similar tricks to set exposure in various situations such as shade, cloud, early morning and late afternoon, etc.
There are times and places where slower, more considered and careful exposure setups are best. But often it's necessary to use the camera's automation to handle situation where lighting is variable or the subject is moving in and out of various lighting conditions. So you need to learn to use the automated modes well, too. (Anyone who tells you to only use M or any other mode is most definitely full of it.... There are times when any of the modes mentioned might be necessary, the only or best way to get the shot you want.)
There's lots more... and it's best to learn it a little at a time and with hands on, real worldpractice. Follow the above suggestion and buy yourself a copy of "Understanding Exposure" by Bryan Petersen.... Might be the best $16 or $17 you ever spend on your photography. Read it thoroughly, then reread it and keep it handy for reference. It's a lot to try to memorize, so just try to learn it a little at a time, and keep practicing. Over time you'll learn to handle most any camera in most any situation.
I didn't mention Auto ISO, which is just another layer of complexity, essentially another auto exposure mode (that lacks any means of Exposure Compensation on most cameras). I also didn't get into the even more highly automated Scene Modes or Green Box that's available on many Canon models. Those take it even farther, automating even more than the exposure with a series of presets, as determined by some design engineer somewhere, that might also dictate the focus mode, type of file being recorded and other factors. For sake of my sanity, I try to avoid these and just use the two manual exposure modes and three auto exposure modes mentioned above. Those are enough to remember, especially since there are also focus modes and color temperature and drive modes and plenty of other considerations to think about setting up my cameras.
But I do want to mention CA or Creative Auto that's on many Canon models now. It's a useful learning tool... A little slower to use but it walks you through setting up your camera via a series of "wizards" on the LCD screen. For example nstead of stopping to think about what aperture to use for a particular shot, CA asks you whether you want the background blurred or would prefer to have deep depth of field with everything in sharp focus, then guides you to adjust for that. If you pay attention to what settings result, such as the apertures the camera is selecting based upon your input in this example, it might help you more quickly learn all the decision making process going into making the image you want. Personally, though I don't feel it's something anyone should continue to use forever, I think CA can be a great learning tool.