The basic controls for adjusting the exposure are actually quite simple. You can use this tool to see what changing the settings do to a photo. Be sure to scrol down to read the brief explanations:

I have no personal experience with Canon flashes, but at the minimum, you want a model that offers at least a tilting head to bounce the flash. A higher model will also have a swiveling head. Canon flashes are expensive; many people have given good reviews to other brands like Yongnuo, which offer the higher end features at very low prices.