You don't need to be the best singer, dancer, or actor to succeed in the theater. You just need to feel like you are, whether you have to pretend or not. You can't go on stage with a bad attitude or else you will end up leaving the stage with an even worse attitude. What I mean is, if you think you are going to perform poorly then most likely you will and you will feel embarrassed when you are finished. To be cast in a musical you must be confident with singing, dancing, and obviously, acting.
             Not everyone can be the best singer but everyone can definitely try to be. It's hard to perform in front of people period, but singing is even harder. This is because there is a fine line between good and bad when it comes to singing. You're either really good or not so good. If you think you're bad, then other people around you will think so too. If you believe that you can sing well other people will think you sing well. You don't need professional help or lessons; you just need your own confidence. Dancing on stage, in front of people can also be harsh like singing.
             Dancing requires physical strength as well as mental strength. Unlike singing, you need to at least have some experience with dance. If you don't, I suggest that you practice, practice, practice. Even if it's once or twice a week, it will help. You don't need to go to classes, just put on the radio and make up some dance moves. When you're onstage dancing, be loose. What I mean is, if you're arms aren't doing anything, wave them around a bit. And if you can't remember a dance step, make one up! It's better than nothing! These things should help you during your performance.
             Here's the big one: acting. I suppose overall theater is based on acting and if you're interested in it you should be familiar with it. Drama is hard work and I think the hardest part is memorizing your script and lines. When you get to larger productions, most likely you won't have to worry about memori...