G'day dementor338,
Welcome to the Outhouse :ernae:
thats a broad question mate, which type of addon do you wish to make?
aircraft
vehicles
effects
scenery
buildings
repaints of aircraft skins
user interface
etc etc
for any 3d models you need a 3d modelling program (most use the free product called Gmax by discreet).
the rest is a matter of looking at the files curently in game and backward engineering how its been done, then applying your new idea's.
any of the list above (apart from scenery this is the hardest part of all to alter, and only a few have cracked it), the rest however is more than doable without any specialists programs (other than GMAX).
give us an idea of were you want to go, and then we can start helping you get there.
regards Rob.