Put a dollar in a jar every time you work out. Set a dollar amount goal and then use the money to reward yourself with something special like new clothes or dinner out. Set a time goal too, like you can add a dollar after every half hour.
You should never do anything today that you can’t do for the rest of your life. So, don't starve yourself or workout for insane amounts of time. Make gradual changes that you can stick with from here on out.