New York City is a major theater town and there are a LOT of classes for various types of acting, including improv class It tends towards young people, but older students are welcome! It's pretty balanced gender-wise. You also don't need any other acting experience.
They usually start with a warm up exercise that is similar/identical to kid's games. Then you are taught the rules of improv - things like "always say Yes and", never reject the premise ("It was all a dream"), mime using the device rather than being the device (pretend to hold a gun rather than shape your hand like a gun), etc. All the while you are given a lot of time to practice and try things out. It's good, safe, fun.
It's a bit expensive, mainly because they sell you 8 classes at once, rather than a monthly set of group classes. But I really enjoyed it. They do offer work study, which can help with the price.
On the plus side, eight classes mean you really get to know your fellow students. It's easy to become friends with them. It's very social, you make friends.
It doesn't take much muscle nor does it require you to work hard. Even if you are not a good actor you can still have a lot of fun. You do put on actual shows - and convince your friends/loved ones to come see them (paying money for the privileged of seeing you too.)
I took classes at The People's Improv Theater: