Aaron - The first thing I would recommend is to read the threads that are stickied to the top of this sub forum. There is a TON of information in there all about the AR platform and it will help you cut through some of the fog.
After you read a few of the threads, come back and ask more specific questions. Believe, you will have them.
At the end of the day, with $900 to spend, you can build a great lower, buy a very good upper and have one hell of a good rifle.