Beer sales are regulated by state thanks to the three tier system currently in place. Find the website for the liquor division of your state's department of commerce. Every state is different. You need a business license and a liquor/beer license that fits your operation. I live in Ohio*, and in order to sell my beer I need a beer manufacturing license as well as a license to sell it on site. If I want to pour tastings, that's another license. On top of all that, it cannot be sold out of my home as the location must be zoned industrial. If I were to sell out of a store, I must go through a state licensed distributor that will likely refuse if I am not already an established business. If I run a brewpub that also sells bottled beer in a store, I'll definitely need a lawyer! Here is the list of licenses in Ohio. As you can see, wineries have it easy here.
Join the Brewers Association to access their much coveted Brewers Resource Directory. If you need legal advice beyond what you can find through the BA, get in touch with one of the many local entrepreneurial resources. Columbus is blessed with The Metropreneur.
It is also important to know that selling beer is not cost effective on small systems. Just ask the guys that resell all of that 3-5bbl equipment. Think big and good luck!
*Ohio has recently changed their laws since this answer, so my licensing comments are a little outdated.