I think it is important for people to come out to the people in their lives for the very reasons seanboe1 discusses.
However, since you are only 18 there are some very practical concerns you need to consider.
I don't know your parents, but you do so you need to be prepared to manage whatever situations may arise. If you are financially dependent on your parents for housing or education, you may need to have alternatives available to you should they decide to break ties with you.
Certainly, it would not hurt to see if any LGBT organizations provide support for individuals coming out as angrygarbagestateresident suggested.
Also there are online resources that provide more detailed information about the coming out process so you have some idea what you may be up against. No one can predict exactly what will happen with your parents and friends, but it doesn't hurt to have some idea of what can happen. Here is one site that provides some guidance: http://glbtss.colostate.edu/coming-out-to-your-parents
A quick google on the coming out process will provide a list of other resources.
Other than that, it is really just a matter of taking the plunge and letting the chips fall where they may. I can tell you that my own experience was easy and without loss of friends or family.