Lean body mass. If you're "fat" then your first step is portion control. Start by cutting all your meals in half, then spreading those halves out. So, if you eat a turkey sub for lunch, cut it in half, eat the first half for lunch, then the next half at 2:30. That should speed up your metabolism a bit and give you more energy at the gym. You'll also feel less lethargic after meals.
Once you conquer that, work on slowly changing what you eat. Things like plain oatmeal and salad with fruit + eggs in place of dressings are hard to get used to but will really eliminate some major sources of calories from your diet.
If you want to hop on clen right away, that's certainly an option, but if you resort to chemicals before you've really changed the way you live your life, you may achieve short term success but it won't last. If you approach it from a "not a diet, but a lifestyle change", you can achieve long term success.
Once you've nailed your diet, you'll realize you don't have to eat perfect 100% of the time. I still eat ice cream (edy's light) pretty regularly. I've just eliminated those weekly pizza runs, heaping plates of pasta, and large lunches.
Good luck in whichever way you go!