i'm 5' one 1/2 and I'm 103 pounds. my BMI [body mass index] is just above normal. I have small & thin arms & legs, but my stomach is kinda thick. I want to gain weight so that I could even out, but if I gain more weight then my stomach will get bigger. on the other hand, I also want to make my stomach flat so I'd have to lose weight for that to happen. I don't know what to do
You're only 16 years old. Things will most likely even out on their own. Just make sure you eat healthily and exercise and reevaluate in a few years (maybe when you're 20 ish) and if you feel that you're over weight then, take some steps to losing weight. You don't sound overweight at all right now and if you start exercising a bit, doing crunches and running, I'm sure your stomach will look more in shape.
I would say do some moderate to intense exercises like 3-5 days a week. I wouldn't try to change too much about your diet because you will lose weight if your energy input is less than your output. As for your arms and legs I would say lift weights 2-3 times a week. Low sets, high reps -say 5 sets of 10.