You should only take fat burning pills if you exercise regularly and have a proper diet. You can take as many fat burning pills as you want, but if you eat junk food and sit around all day, that wont make a difference.
Once you have the fundamentals set up, you can take pills. After you begin taking you have to keep exercising, take a lot of water, eat properly and get rest!
There is a good article about effective belly fat burning pills in this site which you should read for some of the best out there.
Just remember these aren't magic pills!