Yes, you will have to be carefull on what you feed, and how much.
Feeding wet food is also a great way to fatten a cat up! lol
Look for a food which does not contain too much carbohydrates, a lower fat amount, but at the same time not sacrficing too much protien.
You should also encorporate a "work out" plan with your cat, schedual activies as often as possible. Buy interactive toys for them to play with, when you can't be playing with them, buy them a TALL cat tree, so they can jump around on all the levels.
You could also make him work for every morsal of food he gets. Have him do a trick before he gets about 1/4 teaspoon of wet food, another trick for another 1/4...etc etc. You get the idea.
If your feeding him treats, cut back on them, or stop them all together, cats do not have to have treats, unless you have a treat which is in the form of a cat multi-vitamin.
Oh yes, I also wanted to say that feeding the directions (amounts) on the back of the bag/can, are not always what is best or proper for your cat. You can either do what she stated above, or you can start out with the foods recommened amount for her weight, and slowly decrease the amount you give her each day until you start to see some weight loss, but also as long as your not feeding too little.