I'm sort of new at this myself, but I have a friend who I met recently and one night while we were out she just sort of said, "I'm a non-drinker." I said, "Okay, cool." and never asked her why she's a non-drinker. Why do I care why? That is her choice.
So I decided that she's going to be my role model. If someone asks, I'll just say I'm a non-drinker. If they go further and ask WHY then it depends on the situation or person. If it's someone I don't know very well, I might say something like, "My body doesn't react well to alcohol." or "It doesn't agree with me." All of which are true statements.
I think it's also okay to say you're training for a triathalon or pursuing a healthier lifestyle. If the relationship develops, you can reveal more but no need for that on a first date!