I presume that your question relates to pre-marital and extra-marital sex, rather than to the regular marital sex-life. The question of protection/precaution as raised by you gives me that kind of an impression.
In both pre-marital and extra-marital sex, the two most important things would be to prevent undesired pregnancy, and to prevent sexually-transmitted diseases (STDs), the worst of which is AIDS (Acquired Immune Deficiency Syndrome). For both, you should know the absolute necessity of using condoms for safe sex. A girl may opt for birth-control pills, but such pills may only prevent unwanted pregnancy, and can not protect against the STDs.
It is always advised that you try to avoid sex with multiple partners, and try to be as much faithful as possible in your sex life.