How to gain wieght?

I've been trying to gain weight for over a year now. I've gained 15 pounds in a little over a year. But I still don't like my body. I'm in 7th grade. Weighing about 110. Sometimes it goes down to 109 and sometimes it rises. It's annoying when the weight goes down everyday because each day I loose weight after my over a year gain. I want this to stop happening. I'm still too skinny. I'm 5 foot 5 almost 5'6 and 110 lbs. I want to get to 125 for the summer so I can feel confident in my clothes. Tips? Exercises? Foods? Routines?