Talk to your mom if you can, or another adult (preferably female since they will be more likely to understand, but sometimes dads can help just as much.)
If you are sexually active you should definitely go to a doctor or the local health clinic (check the yellow pages). Depending on where you live you can probably go confidentially (without them contacting your parents) if you feel that you aren't ready to talk to your parents about it. If you are sexually active, a missed period could indicate pregnancy, so be sure to go to a doctor if that's the case.
Good Luck.
See Sexuality / Puberty and Crisis Center / Pregnancy
-Erin-