Stomach toning!

So lately I’ve been gaining a bit of weight (I’m 17 and 125pounds, normal and not overweight at all but I used to be bouncing on the verge of 110-115, never weighing a pound over) and it’s mostly affecting my gutt. Any tips on easy, lazy girl (lol) stomach toning workouts? I’m looking for something simple but effective that I could do daily to help get in shape! I run on the tredmill for 15 minutes a day but don’t expect that to help, I do it just to get myself moving as I’m usually a lazy person after long days at work and school. 
Thank you!!