S.T.
First of all realize that at this age most kids get a little "thick" as their bodies prepare for puberty & growth spurts. My duaghter who was always skinny got a little thicker around age 11, then had a big growth spurt and thinned out. She did most of her adolescent growing during ages 12 & 13. So don't get yourself too crazy yet.
At this age I would give my kids crudites while they vegged out on the couch as dinner was being made - so slice red pepper (sweeter than green), carrots, slice cucumbers, celery and a small amount of ranch dressing. That way they had their veggies by the time they got to the dinner table, while they were still hungry and they didn't even realize it. Their tummies were less empty and they ate less at the table.
At age 15 my daughter also began some medication that has caused her to gain weight - and it breaks my heart beucase she's a beautiful girl and I know she'll struggle all the more with her weight as an adult. Ultimately though I can't determine her food choices. The most I can do is make certain foods more easily available. The foods in our fridge: berries (excellent for so many reaasons), bananas (potassium & magnesium), nuts (walnuts, almonds, pecans), cheese, greek yogurt, etc. Try to grill your meats and veggies instead of frying or smothering with butter. The thing is that you want to avoid empty calories and you want to make your calories count. Nuts are higher in calorie than chips - but they are more satisfying and they offer nutiritional value. Also many of us end up drinking more calories than we realize. Juice, soda, whole or 2% milk all have high calories and/or sugar or fat that can add up as the day goes by. We have only skim milk and water in our house and get soda only when we have a party.
Good luck mama.