Replying to Anybody good with sewing?

Ok so I know most of us had to take some what of a sewing class in either middle school or high school and I need some help. I have some shirts but they are really wide but the length is right. I'm reallly skinny so a lot of shirts are too wide for me. I have a sewing machine and have a little knowledge of it but how do I make the shirts skinny and not look really weird? I think you rip the seem on the side but I always have problem with the arm holes. Can anybody help or know of a good website?