Buy a book relating to your camera. But don't waste any more time NOT trying a few things on your own. For example:
P mode: This one is almost a full auto mode, except that you can change the WB as you wish, and a few other things.
Av: remember that using this mode is much like using a film camera on "aperture priority" mode. You control the aperture of the lens, and the camera automatically adjust shutter speed to match the aperture. You still can change a few things manually (change the ISO accordingly).
Tv is just the opposite. You control the shutter speed of the camera, and the camera matches the lens aperture to the speed you have chosen (change ISO is needed). For example, if you are using this mode and the lens aperture is not enough to gather the necessary light for the photo (things will be blinking in the viewfinder to tell you that something does not match), increase the ISO.
A-Depth is pretty much an automatic mode, although you have control on the settings. What this mode helps you with is as follows: selects a depth-of-field for you so that as much as the foreground and the background are in focus. Lots of people don't care about this mode, and I don't disagree with them (I don't use it). But the main reason why I don't use it is because when using super wide-angle lenses like I do for landscapes, the lens itself takes care of the depth-of-field. And, if I am shooting on Av mode, I can control the lens aperture to increase-decrease DOF accordingly.