if it's light yellow/white-ish and doesn't smell funny it's most likely vaginal discharge. your body may be preparing for your periods to start sometime soon, or it can just be the natural way your vagina self cleans (they're really cool like that!). if it has a strange smell maybe see a doctor - you might have a little infection. but it's most likely just normal discharge.
Now that you're coming up on puberty, you will get two kinds of vaginal discharge from now until you're past menopause. One is sort of milky and thin, it comes out about a fourth teaspoon or half teaspoon a day, and dries yellowish or greenish on the underwear. That is your basic self-cleaning fluid for the vagina. The other is thick and clear, like raw eggwhite, and it is stretchy and holds together, and will begin to happen once a month for a few days max. That is your ovulation cervical mucus, designed by Mother Nature to make a hospitable place for sperm to travel when it is going up to fertilize the egg. What you're describing sounds like your basic self-cleaning vagina in action, as breee says. If it is too annoying, you can get pantiliners at the grocery store and wear one in your underwear, changing them when you go to the loo. That keeps your underwear a little fresher.
I think that one of the methods of understanding one's cycle involves charting the days when you have cervical mucus, and when it changes from milky thin to thick and clear. If memory serves from my friends who knew this charting method, a part of the cycle (possibly before ovulation, which would be the first half of your full menstrual month) is a time when the vaginal secretions are much less noticeable or absent.
And yes, having vaginal discharge can be a sign that your periods are coming along pretty soon. If you notice the "eggwhite cervical mucus" (and it can be pretty noticeable, it's so thick that you could cut it with scissors and so clear you can read a newspaper through it) -- anyway, if you notice it, you can expect that means you have ovulated or are about to ovulate. The period follows your ovulation by about two weeks.