Trying to gain weight! Help!

I've been skinny for my whole life, and I really want to gain like 10-15 lbs, I'm 5'4" and just barely 100 lbs. I work out 2x a week to keep some muscle on me and not be a complete stick figure...but it would be nice to fill out a bra a little better and also I've heard putting a little weight on helps with fertility. Any advice would be great!!