How can I improve my singing?

I am a little shy, depending on how well you know me and I don't sing around people seriously very much. I really want to improve my singing though. Through YouTube and myself, I learned to play guitar and would like to be able to sing and play at the same time. I have a little lower voice then most, but I think it is pretty close to average. My mom suggested that I join choir, but I don't know. I am not a teenage music prodigy or anything. Just wanting music in my life other then sports. What can I do to improve my singing?