All women will have some discharge, which is normal and will vary throughout your cycle. White or clear is common, and called leukorrhea. If this discharge is accompanied by other symptoms, you may need to contact your healthcare provider for an exam. An in-office exam can test you for overgrowth of yeast and bacteria, as well as STDs. Yeast infections often cause a thick, white, cottage cheese-like discharge and vaginal itching. Bacterial vaginosis, another common infection, causes a fishy odor and often a thin clear discharge. I am glad to hear you are paying attention to your body. The more you're in touch with yourself, the easier it will be to know when to get outside help.
I can only guess that what you're finding in your underwear is what most women refer to simply as "discharge." It seems that many girls experience this just before their period begins, although you can experience it at any time during the month. It's not uncommon that you get this. When I've had something more serious, like a yeast infection, there is a lot of itching that accompanies it. If you become concerned about the amount of discharge you're finding, or the odor is different from what it is for you normally, you should talk to a healthcare professional.
This could just be your body's natural secretions, so if you're bothered by the smell, just make sure to keep your vaginal area clean (soap and water is fine!). If the odor is different than your normal smell, or if you've never had a yeast infection before and think you currently have one, I suggest talking to your healthcare professional. They would be able to correctly diagnose and recommend treatment for you.