Reddit mentions of Basic Techniques for Successful Fly Tying: A Lesson by Lesson Approach (The Pruett Series)

One other thought: it can be very tempting to pick up loads of feathers and other materials cheap at places like Hobby Lobby. Avoid that temptation. Like the other folks said, but only the materials you need for the patterns you intend to tie. Try not to over-buy in terms of quantity; instead buy small packages of high quality materials. Your stash will grow fast enough that way, and the better quality materials will enable your skills to develop faster since you won't be wasting time and effort trying to get low quality materials to behave and look good. Turning out flies that look good will be more gratifying, and it'll help to avoid some of the inevitable frustrations that accompany the process of acquiring tying skills.

Also, get a book. My favorite is probably still the book by Brad Befus: Basic Techniques for Successful Fly Tying: A Lesson by Lesson Approach, but there are a bunch available now. I'm not saying you shouldn't use YouTube, just that those are more hit-and-miss than the books.