Grad student trying not to gain anymore weight!

Hi everyone. For the past six months I've slowly been putting on weight and didn't notice until I went home to visit friends who told me my butt has gotten bigger! Not a compliment. I haven't been paying attention to what I ate and would eat full meals at 2 am on the regular. I know not good. So of course I gained 7 lbs my first semester. I can't let this continue to happen. I would love some advice and support. After being sedentary for so long it's hard to get up and get right. But I'm on a mission not to gain more weight.