Carol, you don't indicate your age, so forgive me if this is incorrect- if you are a minor and your parents are withholding that info.. that is something we would not be able to interfere with. That is VERY HARD for me to say, since I personally have a family situation involving an extremely nasty custody thing.
An adult has the moral right, in my mind, to know this info.
the answer is, that it DEPENDS on where exactly you are (country/ state), AND if your parents are now living or not. If your parents are not living, you have the right to access their birth certificate(s) which has this info...unless they were adopted. There are a few online birth indexes through ancestry.com. It normally is a fee service.. it can be used at many libraries for free. If your parents have/ had siblings, either their birth or death certificates should also have this info.. depending on where that happened.. you might be able to access those, or they can be restricted to immediate family members (not neice/nephews).
You can send me an email through my profile.. and I will see if I can do anything. It may be possible, again.. depending on the situation.