Start increasing activity
At your level right now, anything will help. If you can run, do it. But start doing all the little things liek parking your car further away in a parking lot, walking to various places, etc. take the steps of the elevator, and so on.
What you should also do is start a resistance based routine, ie start lifting weights. in the long run, it will build bone density and muscle - and building muscle will help you burn calories even when at rest.
