If you use condoms correctly and they don't break, you have a pretty good chance of not getting pregnant. Stick with the more expensive trusted brands, (Trojan and the like), make sure they don't expire, and use lube too (brings down the chance that they will break).
Spermicide is a great option to use along with condoms, if it doesn't irritate her vagina too much.
I would suggest having a back up plan though in case something happens. Keep a dose of plan b around just in case. If she's under 18, she can ask her doc for a prescription or get it from a planned parenthood. If she's over 18, she can get it from a pharmacy. That's always a good idea.