You definitely do NOT want to wait until the heat of the moment. For one, I don't think it's fair to either of you to let yourselves get all worked up and then say, no, we can't do this. Also, it's hard to stop a fast-moving train; you can try to pull the emergency brake, but it won't stop on a dime, if you know what I mean. (If you don't know what I mean, you're probably too young to be here anyway, so beat it.)

No, you have to let him know ahead of time that you won't be jumping into bed with him anytime soon. When you do that is tricky, though. You don't want to do it too soon (like the first date), because it's presumptuous of you to think he's already considering it, even though he is, since a guy who likes you has already pictured you naked, usually the very first time he meets you, and has probably also pictured the two of you engaged in coitus, typically the vigorous and sweaty kind that he has seen in movies where everybody moans and talks dirty and the woman has like 17 orgasms. But he will deny it and you'll be embarrassed that you even mentioned it, even though you both know he's full of shit.

I think the best time to tell him is when you sense that he is clearly moving in the direction of wanting sex. Usually a woman just knows, but if you're not sure, watch for these clues: talking about sex, roaming hands, grinding his crotch against you, pushing your head down toward his junk, dry-humping you from behind when you bend over to pick up a quarter, exposing his penis--you know, subtle things like that.

Of course, you don't say what your boundaries are, and you might be open to certain types of sexual activity short of copulation. That's your business of course--all I will say is that the more you do, the more he will want, and the harder it will be for you to say no, so proceed accordingly.

Thanks for the question.