Many faceters are self-taught, but an instructor to look over your shoulder can be invaluable. If you have the opportunity, find a class. Local gem and mineral clubs may offer something. If you can get to Tucson in early February, there will be classes at the Faceter’s Frolic. See the Community->Events page on this website to sign up.
The best book on the market, in my opinion, is Tom Herbst’s “Amateur Gemstone Faceting”. It’s inexpensive and available from Amazon: https://www.amazon.com/Amateur-Gemstone-Faceting-1-Essentials/dp/3000474749/ref=sr_1_1?s=books&ie=UTF8&qid=1545150474&sr=1-1&keywords=faceting+for+amateurs
There is a second volume with more advanced topics, but the first volume is an excellent introduction.