First thing I have to say is that I admire your bravery. With regards to telling your parents, I think the best thing you could do is calmly explain everything to them. Which parent do you feel closest to? Maybe you could write them a letter explaining that the boy raped you and now you think you're pregnant. If I were you though, I'd wait to take a test first. I'm not sure how easily available they are, but I think that you can get pregnancy tests that should be acurate about two weeks after sex.
Have you reported the boy to the police? I'd also recomend speaking to a GP or maybe a social worker to see what advice they have about keeping the baby safe. Would you parents be able to help you pay or medical bills etc? It might be quite difficult for you to get a job, unless you only apply for temporary posts.
With adoption, would you perhaps prefer an open adoption? (here's the Wiki page xD
http://en.wikipedia.org/wiki/Open_adoption) I know in the UK, you have absolutely no legal obligation to go through with the adoption unless you're completely sure that you want to; if you have the baby and decide you want to keep it, that should be fine legally.
That's all the adivce I have, i'm afraid, having never been in your situation, but if you ever want to talk, you can always p.m. me
`Jessy