Answer:
I have heard that the effectiveness rate of condoms is anywhere between 86%-98%, depending on how properly you use the condom. For more information on this consult the link below.
It is still advisable that your partner get on some form of birth control. Cheaper, confidential birth control can be provided by Planned Parenthoods.
"If you can only use condoms, purchase the ones with spermicide, and also withdraw before you ejaculate." <- This advice was previously given for this question, and essentially it is poor advice. Websites like Scarleteen and also basic high school health classes will tell you that "pulling out" does more harm than good, as your haste can cause slippage, leakage, or breakage. The penis should be withdrawn upon ejaculation, very slowly, holding it at the base so as to assure the condom does not slip off. And Spermicide, when used frequently, can harm the inside of the vagina...
Condoms are 99% effective which means there is a 1% chance of conceiving.
Ok. Before i write I'm goin to tell you that I'm a different person than thoes up ahead and that the Passages above were writin by three different people. They all did give pretty good advice, but me being me I have to warn you of this. Though pulling out before you bf ejaculates is a good idea. There is still seed that leaks out before you ejaculate. This can some times carry enough seed to get the girl pregnant.