Sarah,
The birth control primarily works by preventing ovulation (releasing an egg) therefore conception will not occur. It can also thicken cervical mucus making it harder for sperm to penetrate up into the fallopian tubes and it can alter the lining of the uterus so if conception did occur implantation would be less likely. The withdrawal method can reduce the chance of pregnancy, but there is still sperm present in pre-ejaculation. (Also birth control pills and withdrawal will not protect against sexually transmitted disease).
If taken consistently and correctly the pill is fairly effective. Especially in combination with withdrawal your chances of becoming pregnant are probably very low. However, that said the only 100% way to be sure not to become pregnant is not to have sex. Would you be willing to give life to a child if you did become pregnant? Have you spoken with your boyfriend about whether he would be there for you and your child if you became pregnant? I would think about these things and talk to your partner when making decisions about sex in your relationship. Best Wishes.
Kate