I wrote this a few years ago:
Starting a new business in the USA? Here are things to consider LLC, Corp, Sole Prop
It is worth checking out first.
Basically your choices are to start as a hobby or to start as a real entity. If you start as a hobby then you just start with no real accounting for a profit motive. Once you start making more than a few hundred a year profit you would form a business and do all the right things to start up. Your other choice is the go through the start up motions. The simplest form is a Sole Proprietor and you would file a DBA with your county, put an add in the paper for a month with a fictitious name statement, get a business licensee from your city or county, get a sellers permit from your State and away you go.