What is the best way to get rid of belly fat?

I have gained 2 pants sizes because my belly and only my belly has gotten bigger. I’ve joined a gym, but they don’t seem interested in showing me how to get rid of the belly fat, even though I’ve told the trainer that was all I was interested in. Instead he shows me arm and back and leg stuff and only one little belly thing that doesn’t do anything.

I am about 10 pounds heavier that I used to be. I try to eat right and only occasionally have something not healthy. I drink diet soda and water and stay away from High fuctose corn syrup and trans fats.

Any suggestions??