That "white stuff” in your underwear is completely natural. All women get it at some point in their lives. You said in your question that you haven’t had your first period yet. Well, this is a sign that it is on its way. The white stuff is called discharge and will probably be with you for the majority of your adult life.
All discharge is, is a biological liquid that is expelled from your vagina. It is usually a sign that your vagina is cleaning itself and shows that you are perfectly healthy down there! The amount of discharge that you secrete tends to vary on where you are in your menstrual cycle. If you find that you are producing quite a lot, then it probably means you are ovulating at that your period will be along in a week or two. Discharge is thickest at your most fertile stage in your menstrual cycle. A thin discharge shows you are not ovulating. You may also find that after exercising there is a lot in your underwear, again this is perfectly natural.
However, discharge can give you some signs that there is a problem. In order to check that you are healthy, try familiarising yourself with these signs. If your discharge is thick, white and odourless, this is called your Cyclical discharge which occurs at the beginning and end of your menstrual cycle. This is perfectly normal. If your discharge is clear and stretchy then this signifies ovulation. Now we come to the bad ones. If your discharge is thick and white with the texture of cottage cheese, then this is Candidiasis which could signify a yeast infection or inflamed cervix. Trichomoniasis makes discharge look copious, green and frothy. The sexually transmitted infection (STI) can make your discharge look creamy white or yellow, it will also be odorous. Another STI is Chlamydia which will make your discharge look purulent and malodorous. And finally, bacterial vaginosis will make your discharge thin, with a grey or green colour and a fishy odour.
However, if you have not started your periods yet then there is absolutely no need to worry about the bad discharge symptoms.