Is it stupid to want to join these cles? Ballet, Kickboxing, Karate...?

It's always good to get out and do stuff. And kickboxing? That sounds really cool. Don't be scared to try new things. I was nervous when i started taking guitar lessons (I'm not the most coordinated person on the planet) but I'd say I'm pretty good after practicing. Also, don't give up if you feel you aren't any good. It takes time. And it would also be a good way to make new friends :)