I'm about to start a new project. I'm not a hundred percent sure which one yet, but I know that within a couple of weeks I'll be descending into the dark pit in my basement and I won't eat or shower or change my clothes until I have a book. The first thing I have to discover, before I can begin writing in earnest, is the voice.
The voice, to me, dictates the P.O.V. I know a lot of people might disagree with me, but if the voice in my head is sweeping and epic and has a broad worldview, then my POV is third person. If it's more intimate, more detailed, more insular, then I know it's first person.
Since I dedicated myself to YA I've come to use a lot more first person than I ever did in the past, and there are good and bad aspects to it.
When you choose to use the first person, you have to develop the voice of the narrator. His voice will define how your book goes. My narrator in The Deathday Letter, Oliver, has such a unique voice that it shaped how the book moved. It shaped the descriptions, it shaped the way things were viewed. Your first person narrator is the filter through which the story is told.
That's an important concept to grasp because too often I read books with boy narrators who describe minutiae that most boys wouldn't recognize. If your narrator is a typical teenage boy and he runs into the girl of his dreams, he's not going to be describing the glint of her eyes or the curl of her hair or the sassy boots she's wearing. The chance that he'll even know what color her eyes are after the encounter is slim to none.
Not to say that those details aren't important, just that you have to get to them in different ways. If the color of her eyes is important to the story then during their encounter you can have your girl hold up an outfit and ask your narrator if he thinks it matches her eyes. She could even throw in a little snark by saying something like, "Those things about 8 inches above my breasts."
You get the point. Using the first person can seem like a much freer way or narrating because you can run a little wild and throw in come crazy stuff, but just remember that everything your narrator sees and does and says has to be in character. If it's not, it'll come off as false and your reader will know it.
That's why I need to know the voice before I can begin. I need to know if he's a jock, because then he might have a tendency to describe things in terms of sports. Of if he's a bookish sort, he might have a favorite book he quotes a lot. Or he might just be kind of oafish and have an eighth grade sense of humor and turn everything into a penis joke.
So that's something to consider when determining whether to use the first person or not. Do you have a narrator with a strong voice and a strong point of view. If you don't, you might either want to rethink your narrator or rethink your point of view.