When used perfectly, male condoms prevent pregnancy in 98% of cases, but this is closer to 85% in real life. Failure rates for female condoms tend to be slightly higher than those of male condoms.
Other contraceptives such as the pill, an IUD, or an implant, have lower failure rates, and you may wish to consider adding another one such as the pill if preventing pregnancy is very important to you.
Condoms are also 98% effective at protecting against common STIs like chlamydia and gonorrhoea, and are 85% effective against HIV transmission. However, condoms don't completely protect you from herpes, genital warts and syphilis which can be spread from skin-to-skin contact.