Welcome aboard, Terry13.
We're always glad to have someone new to push around, I mean to guide and encourage.
Now go touch up the scratches on the bottom of the hull.
There are some good guides at the sh4 forum
http://www.subsim.com/radioroom/forumdisplay.php?f=202
Any questions, ask away. We all had questions when we started and are more than willing to help.
The mods can get confusing. Most of us start with a "supermod" that makes a lot of major game changes. The starters are Trigger Maru Overhauled (TMO) and Real Fleet Boats (RFB). The next supermod is Run Silent Run Deep Campaign (RSRD or RSRDC) It changes the traffic and battles to historical accuracy. The last is Travellor's mod and Optical Targeting Correction both which make other major changes also. There are many smaller mods that change individual aspects. My best suggestion is to read the info with each mod and decide what you want. There is another mod called Operation Monsun. It creates an Atlantic, German campaign for sh4.
When you download the mods, save the files in a subfolder in your documents folder. When you unpack the mod folders, then put the unpacked files in the directory that JSGME tells you.