What is a good diet for a 14 year old that wants to get a flat stomach?

I weigh 120llbs

this is how I eat

morning: either a slimfast or protein shake

lunch: slimfast and a couple slices of cheese and some cucumbers

when I get home I have: a little snack thats healthy

I walk on the treadmill for 10 min then run for 10 min

for dinner: vegetables and fish
my height is around 5’5
I don’t know if you would count this as a exercising but I have horses and ride like 4 days a week, not trail riding, training horses. I use alot of my leg muscles and lower body