the earlier you tell your parents, the better.
they will have more time to accept the new life coming into the world, instead of you waddling in with a big belly and them putting two and two together.
Reptar gave you some good advice, like bring your boyfriend, a best friend..you could even tell your boyfriends parents first, and they could come with you to tell your parents. but like i said, the earlier the better.
My sister inlaw gave me that advice when i found out about my pregnancy, and in the long run, it helped and im glad i did it sooner than later.
My parents were disappointed, upset, angry..for alittle but they have come to accept this now.
Just stay calm when you tell them, dont fight with them, listen to what they have to say, and MAKE SURE you have a plan, because they WILL ask you that. Like what you are going to do, how you will support the baby, all questions like that. If you can answer all their questions honestly, it might make them accept it alittle more because you two are being responsible now, knowing what has to be done, and taking it from there.
Talk to your doctor, and pray for the best, plan b will not do any harm to your baby, its just hormones, and you body gets a TON when your pregnant. lol
If you feel like this is going to halt things, or be too much, you do have an option of adoption, if you would consider that..its not a bad thing, but another option for you.
Remember you can always have a baby when you feel you are ready. but thats just an option, like Reptar said, many women on here have succesfully raised their babies as a teen, and many are about to as well, they can give you support and possibly even the courage to do so.
good luck