Condoms are one of the best forms of protection against pregnancy. They also protect against Sexually Transmitted Diseases, which makes them doubly useful and good for you.
The best form of contraception, statistically, is combining condom use EVERY TIME with birth control pills. That's about the safest you can be, in terms of pregnancy.
Bear in mind, though, that all birth control methods have some failure rate. You need to discuss what you'd do in a worst-case scenario, if your girlfriend DID become pregnant despite taking precautions. Hopefully you'll never need to deal with that, but it is better to be prepared than to be caught unawares.
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