First time at the gym... some advice?

I have decided to go gym for the first time in my life but I don't wanna look like an idiot and I wanna loose weight and gain muscle but I don't know how to go about it and what to do when I am in the gym any suggestions? Im pretty fat and i was wondering if im gonna look like a fat kid and will people stare at me. I have never stepped into a gym. so i have really no idea. i also have low self esteem and stuff. So. what are gyms like. Are their other fat people there as well trying to loose weight. :[ pretty nervous. also im VERYYYYY lazy...but i really want to lose the weight and really fast because summer is almost over and i wanna look good at school (starts in 1 month)......so please help me! I'm 16