Quite honestly, the easiest way you're going to learn to mod is to download Pakscape and then go to:
http://medalofhonor.filefront.com/files/Medal_of_Honor/Spearhead/Mods;1571Look around at their mods on that site, then download them and open them in pakscape. Say you're looking for a rifle only mod...Download one, and open it in Pakscape so you can see what other people have done. This way you can reverse-engineer it that way.
Or you can do it the hard way (the way we did it back in the demo days before the full version was out lol), and use Pakscape to open your .pk3 files from your game itself. Then you can go through looking at all the files and figuring out what each is and does from there.
I wouldn't suggest adding things to maps until you can figure out how to properly mod weapons first. It will get messy. If you need any help with the coding or anything, feel free to post up clips of code that you work with and I can point you in the right direction.