Hello,
I got my games form Steam too, so I think I can help you.
First of all, download the JGSME ( i believe someone up here told you to do so...)
Ok, to install mods, you'll have to find the game on the Steam folder. It's usually located on C:\Program Files\Steam\steamapps\common\silent hunters wolves of the pacific
Once located, extract the JGSME files to this folder. This is the "root" folder for SH4 using Steam.
Follow the instructions of the JGSME Reame file.
Two folders will be created in the SH4 "root" folder: MODS and !Backup.
You can download mods anywhere on your computer, but when extracting the files, be sure to point to the MODS folder (the full path is something like this C:\Program Files\Steam\steamapps\common\silent hunters wolves of the pacific\MODS
That's it! There you have it!
Recomended mods are TMO 1.9 (and its patch) and Easy AoB by Mr. Nicholas. The first one is a general remodelation of the game. If you are familiar with SH3, I strongly recommend it. If not, stick to the stock game until you gain confidence and experience.
It can be found here:
http://www.subsim.com/radioroom/showthread.php?t=132330
The second is a mod for easier determination of a torpedo solution.
Can be found here (be sure to read the entire post):
http://www.subsim.com/radioroom/showthread.php?t=152945
And look for the Stick posts on this forum regarding tactics, technical stuff and how to's.
And last, but not least...welcome to this (somewhat) crazy community!