DO NOT USE 2 CONDOMS like Die666 suggested. One condom is better than 2. Condoms are not designed to rub against the same texture. A lot of condom breakage are due to people wearing multiple condoms.
You can't be 100% sure that you wont, unless you quit having sex all together. I think you and your girlfriend should talk about if you really want to have sex and if you do then you both need to take precautions. Now if you are sure about having sex then you should wear a condom, she should be on birth control and maybe use some spermicide. They actually make condoms with spermicide on them now. Good Luck!
hmmm two condoms birth control and pulling out would probably be a deffinate.
The only way to know for sure is abstenance.