I know everyone else has already said this, but...
I think it's best to just force yourself to do it. Of course when you're ready, just be sure not to back out. And if it's hard to just come out and say it, just do something stupid, or mumble something so they question it. (It's what I do when I'm trying to force myself to say something. It makes it easier to admit some things-- for me at least.)
People all have different ways to spit something out, but the sooner you do, the better.
Besides, it's best if you tell them aside from them just finding out. Being honest can show that you trust them and can be completely trustworthy... and that you're brave enough to come up with the truth no matter how hard it may be. And by holding it back longer, it may hurt them a bit more. Like a, "Why couldn't she just tell me? Doesn't she trust me?" kinda thing.
If your parents love and care for you, they'll accept it.