This is often a situation which is best handled with care. There are no exact directions that I can offer to you, because how you and your family deal with this is based on a number of things: your age, your community, your relationship with your family, your previous religious experiences, your parents' commitment (or lack of) to a certain religion, and their willingness to explore new ideas.
Although it seems like a wacky idea, many people advise teens that it might be better to wait six months, a year or more to tell them about your Islam. The reasons for this vary: you need to be firmly established in Islamic practices (such as regular prayer), and you need time to make friends and build a support system within the Muslim community. These two things serve as a protection for you if your parents react by attempting to "deprogram you," or schedule "an appointment" with the local minister / priest / rabbi. In such a case, God willing, you will be able to rely on your knowledge of Qur'an, and the strength that being a practicing Muslim has given you. Allowing yourself time to build a support system within the Muslim community is important so that you will have friends to help and guide you, to help answer any questions or concerns your family might have, and to help you out should your parents decide that you can no longer live in their house.
If you are fearful that your family may react with physical abuse, or a kidnapping and "deprogramming" attempt (yes, it happens), please make sure that you have someone there as a witness and support. Whether you are Muslim or not, you have the right not to be abused. If your family is abusive towards you, seek the necessary help to get out of that situation as soon as possible.
Another reason that it might be wise to wait awhile is to allow your parents to see the positive changes that Islam will bring about in you. For example, they will see you giving greater care to hygiene and appearance, and possessing greater discipline in your daily activities and your schoolwork. They'll also the fact that you are not falling under negative peer pressure to drink or drug or have sex, and that you are more willing to honor your parents by helping around the house, etc. Allow them time to be pleased with these positive changes, so that they will understand that these positive changes are one of the benefits of Islam. If they see that Islam is "good for you," they may react more positively when you talk with them about it.
(Peace be upon you)
