Gaining weight

I’m struggling to keep my weight, like I have my times when I have a loss of appetite but I am a foodie trust me, every 5 min I go look for a snack, I’m also starting gym again and I need to eat double what I lose or else I won’t gain, but I don’t know how to keep to it? Make meals that I like? Set a goal to eat everything off the plate? I’m also at school atm and for lunch I usually just eat a wrap because my options are limited, and tips would be great