OK, what you'll need to do is:
1. Unplug your PC from the power socket and press the power button a couple of times
2. Open your case up - should be a couple of screws at the back of the case you can remove to take the side panel off (on the left hand side as you look at the front of the PC)
3. Have a look inside, and measure the width of the CPU fan (will likely be 92mm, 120mm or 140mm)
Feel free to post a pic on here and we'll be able to help you out. Might also help if you confirm if you're adding a fan to the CPU cooler, or just to the case.