I went to Pennsylvania Gunsmith School (PGS). Besides teaching me the basics, I learned that there is always something else to learn, and that no gunsmith can know everything at all times... notes help a lot! I have 6 large binders filled with hand written and typed notes, as well as numerous books (such as the Gunsmith Kinks set and all of Kunhausen's works, among others), and I know I will probably fill out another 6 or so notebooks in my career.
You should check with your state for their requirements.. For example, in NY you are supposed to register as a gunsmith with them for insurance and liability purposes... all it takes is a signature saying you are operating as a gunsmith, and that all work you do will be covered by your business insurance.... means all work you do, including free stuff, is a potential lawsuit if something goes wrong.....
I know NJ is the same, but CA and PA don't require anything other than a business license and the appropriate state/federal paperwork for being a gun dealer.