My reflex is always to begin writing in first person. It's only been in the past few years that I consciously stop myself to ask if it's truly the best POV for the story. I had great fun writing in 3rd-person omniscient for a novel draft a few years ago, but overall I couldn't maintain enough interest in the story to spend time revising it. The novel I'm working on now (3rd draft) is in 1st person, though I did have a moment of misgiving about the POV during the second draft. I finished the chapter, opened a new Word document, and started rewriting the chapter in 3rd person. I rewrote about 2000 words that way before realizing they didn't have the same flair. I went back to 1st person with a sigh of relief.
If you have a beginning in mind, or a pivotal scene that you know will happen at another point in the story, try writing those from several POVs - 1st person, 3rd person limited, and even 3rd person omniscient.
I will say that for contemporary YA, 1st person tends to be popular (with an ever-increasing number of books written in 1st person present tense). That's what I preferred to read when I was a teenager. For YA fantasy, 3rd person seems to prevail, though 1st person is becoming more popular for YA urban fantasy.