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.
I got the Canon T3i a couple months ago. Up to this point I have been using a Sony DSC-H9. I've been able to catch beautiful pictures, but wanted to upgrade to my first DSLR because I love taking pics and want to 'step it up a notch'. I have read the manual and have took great pics all in the auto mode. I also bought a 'Canon T3i for dummies book' and read that as well. Is there any particular book or dvd that you suggest for a first time DSLR owner? I am nervous about messing with the settings! The manual it came with sucks in my opinion. Also, we are going to Hawaii for the first time and I would also like to purchase a flash. What type of flash would you recommend? I love outdoor and nature photos, but keep in mind I have 4 boys and they are in lots of sports, I also like walking around and finding stationary objects to get a pic of. Thanks for any advice, I can't seem to figure out all that mumbo jumbo about aperature etc!