Join PPFA and start buying books from their bookstore. You can educate yourself that way, and PPFA has a great recommended reading list as part of their certification program. Even if you don't want to go for certification, the reading list covers most of what you "gotta" know to be a good framer. You still need practical "hands-on" experience, though.
Get a job with a local frame shop and learn everything you can from those who work there. Be careful, though. Some long-time framers have old, bad habits that just carry on from one generation of framers to the next that way. You still need the books to verify, supplement, and update what you learn in practice.
Attend trade shows and take the courses offered by whoever is sponsoring them. PPFA's "Framers Palette" program is popular -- Affordable classes and very helpful.
There are several framing "schools" -- usually suppliers that offer classes on some schedule. You can learn more about them from PFM or Decor magazines.
Home-based framers are everywhere. Some are very good framers who take their work seriously, and some are not. Most of them started like you -- interested but uninformed. Please, please, please...if you want to get into the framing business, learn how to do it right.
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James Miller,PPFA-CPF; PPFA Certification Board Member; FACTS/GAFP Committee Member