Discuss your options with the father of the unborn child. He should have a say in what you are going to do, and half the responsibility.
There are essentially 3 choices that you have: 1) Keep the baby and raise him or her, 2) Put the baby up for adoption, or 3) Abort the pregnancy.
If you are thinking about having an abortion, I encourage you to tell your parents before you have the abortion because it will be their grandchild and they may want a part in raising it, or at least may want to offer their support in whatever you decide.
Also realize that there are downsides to abortion that you have to think about first - this isn't a decision to be taken lightly.
If you decide to have the baby, then you need to tell both your parents and the father's parents because then you can see what your options are as far as supporting and raising him/her goes. Ask them to help you in whatever ways possible.
If you don't want to keep the baby though, or feel that you can't take care of him or her once he/she is born, but you do not want to have an abortion, you can look into long and short term adoption programs.
The best advice I can give is to call one of the resources below (if you are in the U.S.). Your calls and visits remain strictly confidential and they can discuss in depth all of the options available to you. They can also provide support about telling your parents and stuff.
If you are not in the U.S., visit some of the sites for information about help in your area.
I wish you the best of luck and support in whatever you decide is the best decision for you.