How long would it take to get a flat stomach?

I’m starting to run 20 minutes everyday and doing 20 sit ups. Is this the right thing to do to get a flat stomach? My stomach isn’t huge but not really where I want it to be. I’m 14 and weigh 121 pounds. I have these love handles ( I think thats what their called) too. If I am taking the right steps how long till I see results?