great, glad to have a new modder here
if you like u-boats that side of the game mostly uses the sh4 ATO Mods forum (atlantic theater operations)
found here:
http://www.subsim.com/radioroom/forumdisplay.php?f=234
and most uboat mods are found there, this is usually US side fleetboats and mods designed for them.
that way you know where to look for things.
dont forget to check out Mobius22's thread listing lots of mods for sh4, find it here:
http://www.subsim.com/radioroom/showthread.php?t=140742
and welcome to the forums and modding sh4