First off, you need a tool. The best one around so far is Timeslip's
FOMM. I won't go into how to create a new plugin and cut straight to the chase. The entries you're looking for are DATA and DNAM.
DATA looks like this:
Code:
F4 01 00 00 90 01 00 00 00 E0 40 0B 00 18
The numbers you're interested in are the 90 (weapon health), 0B (damage), and 18 (magazine capacity.
These numbers are in hex. If you don't know hex, don't sweat it. Just open up your Windows calculator, punch in the number you want and click the 'hex' button and it will show you what that number is in hex.
Mucking about with the DNAM is more tricky, but this is where you can tweak real-time accuracy, rate of fire, zoom, and a whole host of other fun stuff. I would have to write a proper tutorial to explain what was what in there and most of us are still figuring it out.
Hope this helps a little bit.