You'll need a mod folder, for one thing. Make a folder in .../FreeSpace2/ called "MyMod" or something. What you'll need to do is make a tbm file in .../Freespace2/MyMod/data/tables/ called "*-wep.tbm". The ".tbm" extension tells the game to add to an existing table file, and the "-wep" tells the game that it adds to the file on weapons.
Then, start out with this:
#Primary Weapons
; Your weapon code goes here
#End
All the fields for weapon stuff are found
here. Some fields, like all the "Laser" fields starting with '@', "$Model File", "$Piercing Impact *", "$Trail" and its subfields, and "$Pspew" and its subfields all modify what the weapon looks like- usually, by giving the name of an image file stored in FS's VP files.
If you want, I could do several tuts on weapon modding. I have some free time
