First Time

So I'm 16, my boyfriend and I have been a thing for 3 months and we both wanna move on to the next step... I really don't want to get pregnant and I am super paranoid. I bought condoms with my best friend yesterday who also is in an intimate relationship. But I haven't done it before and he has. I'm scared for what it's going to be like, and this app tells me I'll be okay and only 3% fertile on the day I wanted. But still, I'm so scared. Has anyone ever saw on the app that they'd be okay, then they got pregnant? Help.