Do u think im underweight? and what swimsuit would fit my body best? thanks (:?

As long as you're eating healthy you're fine. You most likely just have a thin body type and a fast metabolism. And for a swimsuit you should definitely go with a top that has ruffles. It gives the illusion of a fuller chest. Or you could try one of those bandeau tops. Most girls with bigger chests can't pull them off but you definitely could, and they're really cute!