15 years old girl, face red, warm to touch, a little dizzy, what's the problem?

Warm red face. Need more information. Perhaps the warmth is from a fever. Get a thermometer and take her temperature. Have her sitting or lying down so she doesn't fall. If she has a fever then she is coming down with something, so give fluids and see how she does. If you get more information ask again. If she gets sicker then call your doctor.
More info needed. How long have symptoms been there? What's her past medical history? Medications? Drug use? Menstrual history? More information is needed to answer your question more specifically.