Well, hopefully your parents will support you, even if you are doing something they don't agree with. Being honest and truthful is the best way to get in life! Obviously they might be shocked or have mixed emotions but in the end they will be ok about it, after all it is your choice of what you are. It's the same as religion. If you wanted to be Buddhist would they be ashamed or disappointed? No, because its your choice. Hope it helped :D
You need to ask yourself a few questions before you come out to your parents:
Are you dependent on them? If so, will they still care for you or kick you out if they find out? If you answered yes to both of them, now is not the time.
Will coming out compromise your safety? If so, now is not the time to come out.
Lastly, do you feel it is time to come out based on the above answers and your personal comfort level with the idea? If so, now may be the time. Consider it strongly before you just come out.
Best of luck.