This subject has been covered extensively. A quick search and you will be likely get the whole story. Also you need to check out AR15.com. They have a whole forum on building ARs. They also have instructions for assembling a lower and links to other sites that allow you to download military armor's manuals and such.
If you are trying to save money by building an AR, forget it, you won't. Unless you really shop around and get some really fantastic deals. If you want to know your weapon inside and out, if you like to tinker, if you are looking for a fun project, if you take pride in doing things yourself, or if there is an AR configuration that you don't see made by any major manufacturer, then jump on in, the water is fine.