Sex, you say?
Well, I'm not particularly knowledgeable in that area, I've only had sex with two different people(one of them being my current, absolutely lovely boyfriend, and a douchebag before him).
For me, sex is an act of showing real trust and comfort with another person. It is not something you should do just for the sake of doing it, or because you just feel obligated to do it. A lot of people seem to have the impression that sex is a "must" in a relationship, as they seem to jump into it fairly early. There's of course nothing wrong with it, if you can do it early on, or if it takes a year, doesn't matter, but you should be sure about it - regrets for something like that can be devastating for your mental health(especially if you're young).
Something I think is very, very important with sex however is precaution - use fucking protection(pun intended). I have a neighbour, she had a kid at fourteen and the guy she had the kid with of course left her shortly after she gave birth. And do you know what the most horrible thing about it is? She didn't tell her mother until she was in the 7th month.
Now, I absolutely don't look down on casual sex, some people are able to enjoy that just fine. But for me personally, I wouldn't be able to be that intimate with a person I didn't fully trust and know, however.
My boyfriend is epic by the way. No more details.
EDIT: Oh, right, these opinions are based on personal experiences and a LOT of observation(I hung out with people who thought that getting piss drunk around strangers ten years older than them and having sex behind their parents back was a great idea, and that there was nothing at all that could go wrong).