The main way to prevent STDs is while using a condom. Most people know that.
However there are also female condoms that are not so popular but work.

Make sure your condom is latex (if you are not allergic to it). It has been proven that it more safer than natural condoms.
Condoms act like a small “ barrier” from the penis to the vagina. It keeps the blood, semen and vaginal liquid separate. However condoms cannot stop all STDs from spreading.