This is a problem that most Wiccans (or any other type of pagan) have to face. I myself did, as did many of my pagan friends. When you are comfortable enough with Wicca to call your self a Wiccan/Witch, you are then faced with the problem of how you are going to tell your friends and family. This is often referred to as "coming out of the 'broom' closet", and with good reason.
If you were raised by a family who is very strongly Christian, or any other monodietic religion (I think I made that word up), than chances are, that you will not get a very good response when they find out that you have an altar set up in your bedroom to the Goddess Diana or the God Pan. In many cases, as it was for me, the best thing to do is often not to tell your family at all. That is unless you are pretty sure that they will not condemn you or try to roast you for dinner.
Telling your friends can be just as hard as telling your family. Chances are, you have a friend or two who is really religious, even though you might never be able to tell. These are the one's you have to be careful with. I have a very close friend who is strongly Christian, and I had a hard time telling her. To this day, I know that she doesn't 'approve' of my religion, but I think that she accepts it as a part of me.
I guess that the best advice I can tell all you closeted Wiccans out there, would be to use your best judgement. I might suggest not telling anyone until you have enough knowledge in your head to be able to explain what Wicca is, and what your beliefs are. Nobody (including other Wiccans) will take you seriously if you run around claiming "I'm a Witch, so don't make me mad, or I'll cast a spell on you!", these are the types of people who give us a bad name.