Gaining Weight

This will seem a bit off because usually people post and ask ways to lose weight but I'm curious to know how to GAIN it. I've always been tiny, maintaining a weight of 100 but now I can't break past 89 pounds. I've started calorie counting so I can intake more calories, but I was wondering if anyone else had any good tips to help :) I am planning on getting my thyroid checked out to be sure it's not that, but what are some things I can do while I wait?