Jack said it right. Using M mode is best, but if the ambient is changing and you want in-camera metering to adjust your exposure you could go into Av mode, set the aperture and ISO you want, then dial in -2 stops (or thereabouts) of exposure compensation (not FEC, just normal EC), then the flash will light the subject and the camera will expose for the ambient at 2 stops under. The reason for underexposing the ambient is that if you don't you will get "ghosting" from subject and camera movement, but when the ambient is underexposed a fair bit the trails from the shake will be unnoticable since the flash exposure dominates them. Of course this only applies in poor ambient light where you are at a slower shutter speed than you'd usually like - in good light you can just "balance" the exposure and expose normally for the ambient, using the flash to fill in the shadows. Another creative option you may not have considered yet is to try grossly underexposing the ambient outdoors in good light (like -3 stops or more) and then using the flash to light the subject giving you a properly lit subject against, for example, a really saturated and moody sky.
So EC adjusts the ambient exposure, and FEC adjusts the flash exposure - simple and elegant once you get the hang of it, and it gives you two different regions of exposure to play with within the image giving a lot of options.
Again though, unless there is some good reason not to (like fast changing light) do what Jack said and use M mode.
Also, if you are new to flash, do the other thing Jack mentioned and practice bouncing it off surfaces both in and out doors - don't just limit yourself to the ceiling as so many newer flash users seem to (I used to), think about all your options - often bouncing off a side wall or a corner in a room gives light that looks much more natural since it is slightly more directional but still softened by bouncing it. When you are bouncing be aware that the flash is not only being spread out by diffusing off the wall/ceiling/whatever, but is also having to go further, so to avoid working the flash too hard you may want to up your ISO a bit or open up the aperture, or both.