I don't think anyone does truly know how to begin a project when they're first starting out. I remember back in my ultra noobie days, I'd follow along with a dozen and a half tutorials, but when it came time to do my own stuff...I was stuck. I didn't a workflow yet. Didn't know how to make that first step. You can watch a thousand people do it a thousand times, but until you do it yourself, you really don't have any idea of what to do.
So...you just do something. Don't think about how best to begin, just pick something that seems kinda interesting to do, and start doodling around. If you mess up, or the results aren't as good as you'd like, then scrap it and start again. After awhile, you'll start picking up on the ebb and flow of things, and you'll start improving. Keep at it, and a couple of months later you'll find yourself amazed that you're handling the stuff you used to spend hours struggling with with barely any thought at all.