Depends on what you want to do.
Editing a subach to do 50000 damage is pretty easy for example. Importing a self made 3d model is a whole other can of worms.
First off, the wiki if your friend!
http://www.hard-light.net/wiki/index.php/Main_Pageit has a ton of info on modding, tables etc etc You can also get a bunch of tools required for modding from there
http://www.hard-light.net/wiki/index.php/QuickVP for example can open .VP files, so you can extract .tbl files (
http://www.hard-light.net/wiki/index.php/Tables) from it, which you can then easily edit with notepad.
Make a new folder in your freespace 2 map (c:\freespace2\mymod\ for example) and place your edited files in it. .tbl files go in mymod\data\tables.
Select your mod folder with the launcher to test out your changes.
If you want to make a new mission with your mod (and you probably want to

) you'll have to make a FRED shortcut and add -mod mymod (or whatever you name your mod folder) at the end of the target path.
Hope that sets you off on the right path