Well, what's your position? What is the likely reaction of your family and friends? What is the emotional cost, to you, of keeping silent? How much do you have to hide from people? What is the general attitude of your community to LGBT people? Coming out is a deeply personal decision based on many factors; the list above is by no means exhaustive, but it is a good starting point.
As Liam said, it'll depend on a bunch of factors. Only other thing that will factor in is your age. Because, sad to say, people are more likely to consider you "confused" or "just going through a phase" at age 13, even if you've thought about this for years, and came to the conclusion quite some time ago. If you think people will be supportive, you can at least start planting seeds. You might say something like "I honestly don't know if I'm straight. I'm going to give it some more thought, and some more time, and see what happens." This will make it clear that you're not being impetuous, that you're carefully considering things. Or, if you'd rather not be that misleading, you can let them know of your thought process in the past. "I guess I first thought I might be 'different' back when I was ten, when I noticed such-and-such. Over the past few years, I've noticed this and that and the other." Lex