Plan B is tricky. What it does is prevents ovulation, just like the regular birth control pill does. It just does it in two doses, rather than daily ones. However, if you have already ovulated, Plan B will have no effect. They say that 7 out of 8 pregnancies can be prevented with it if it is taken soon after unprotected sex.

Since you took it so soon, you will probably be okay. However, why don't you go to your doctor and get birth control pills? They're so much better than worrying like this in the future.