How do I ask for a cocktail “light” or with less alcohol?

Just turned 21 this past Sunday, and alcohol is this massive unexplored landscape to me.

I have had a few drinks before turning 21 at home, but very rarely (think maybe 3 times per year). Now being 21 I am suddenly curious, but don’t want to be the ignorant youngster at a bar or restaurant that has no idea what he’s doing. (Not sure if this matters for recommendations, but I have been blessed with a higher-than-average paying job for a college student, and thus can bypass all the college bars and head to some nicer places for drinks, fancy cocktail recommendations welcomed!).

After doing some research and learning how to measure alcohol (wanting to avoid drunkenness), I think I like the idea of an old fashioned, but I’m curious how one would order that with less bourbon (or any cocktail with less alcohol) Looking for maybe 1 standard shot as opposed to the shot and a half or 2.25 oz standard for the old fashioned?

It’s entirely possible that I am slightly paranoid about it and if that’s the case let me know!

Thanks in advance.