This white cream substance on your underwear that you are describing sounds completely normal and is called vaginal discharge. This begins at the onset of puberty and is your body's natural way of cleaning and moistening the vaginal area. This is due to an increased level in hormones that also happens as part of puberty. You will probably notice a change in your vaginal fluid as you go though each menstrual cycle (thickness versus thinness, increase versus decrease).
Vaginal fluid also increases during sexual arousal whether it is due to a dream, masturbation or sexual activity. Again, this is completely normal. This provides your vaginal area with natural lubricant to make sexual activity more pleasurable.
Having said all this, if your vaginal fluid has changes in color (greenish or yellowish), odor and amount or becomes itchy or irritating, it could be a sign of an infection. If this is happening, you need to seek medical attention to be diagnosed and treated, especially if you are sexually active.
Going through puberty can be difficult because there are so many personal changes happening in your body. You need to find someone that you can trust and talk to about your concerns like your mother, an aunt, doctor or school nurse.
If you are concerned about these changes or think you may have an infection and cannot talk to a trusted adult, please call a local teen health center or Planned Parenthood in your area. In New Jersey, please call the Adolescent/Young Adult Center for Health at (973) 971.5199. A medical professional at a teen health center will be able to answer your questions. Always, feel free to submit another question to us.