I want to ask this guy out but I don't know how?

So one of my best friends, whom I've liked for almost three years, actually ended up asking me to homecoming this past year, and we also went to prom together. We hang out quite a bit but it's usually in a group, since we have the same friends (and we're both pretty shy). I've wanted to tell him for a long time that I like him and want to go out with him but I just get so nervous, and don't really know how to go about saying it (or when to say it). I'm pretty sure he likes me as well but I don't want things to be weird if I'm wrong...

He and I are leaving for separate camps this week and I'm really tempted to call him and tell him my thoughts but that doesn't feel right.

What's the best way to go about asking somebody out?