Finally bought the A57

After 2+ years of reading forums, reviews, etc. I finally bought the A57 with the 18-270. I've got a lot to learn. I was at my granddaughter's birthday party which was indoors and the lighting was metal halide. I'm not sure which exposure to use....Auto, P, A, or manual or which white balance. Should I shoot with each of them until I get the shot that looks right? Should I use auto ISO or ???? I also bought Dave Busch's book which should be arriving any day now. Thanks for viewing.