When you want to have sex for the 1st time, you should first get tested for HIV. Even if you are both negative you should use condoms for the next 3 months until you are tested again. In those 3 months (the window period) you could have HIV but it doesn't show on the test. If either of you has sex with someone else, even once, you have to test again, use condoms, and test again in 3 months. You both need to be honest and mature, slipping up just once puts you both at risk.
Attribution: Straight Talk Foundation & Marie Stopes