Am I average weight for my height?

I am 5'7" and weigh 115 lbs. My weight goes up when I eat, but then it goes back down (of course) Is this a normal weight? My bff complains that she's fat but she's 5'3" and weighs 85 lbs so I feel really fat around her. How can I lose weight? Do I need to lose any weight? I've checked my bmi numerous times and it says I'm normal but I checked my friend's bmi and it says shes underweight. I want to be underweight too...