This video goes over the entire process of assembling a basic 16" AR-15 carbine. The AR-15 is ubiquitous with American firearms ownership in general and many people are interested in building their own rifle to save a money. With a few simple tools, a little instruction and quality components you can build a complete rifle for less than $500 in some cases. .
There are several companies that produce stripped uppers and full "build kits" (less lower receiver) in many different configurations to suit your needs. There are also several specialty tools shown in this video from Brownells and Geissele Automatics which make the build process much as well as maintenance and upgrades much easier down the road.