How do we know if either one of us has some type of STD?

Hi. I've never really used this site before but I have a serious question. So I want to give you some background info first. To tell you now, I am gay and my friend is too. We are also both 16 at the moment and have known each other for almost a year now. We've been wanting to have sex but we kind of have a barrier, that being I live in California, he lives in Oklahoma and we probably won't be able to meet for 2 years (I'd like to take the time in explaining everything but you most likely don't care) The thing is, I want to be able to know if unprotected sex is safe between us, like how do we know if either one of us has some type of STD? I'm just trying to be as safe as possible because we both have never done anything like this before and want to be able to do whatever. Just overall we want to know if unprotected sex is safe. Sorry this is long but I tend to go a little overboard with stuff.