Welcome Mr. Gord! SH4 is a fun sim good choice! I am not the official welcoming guy but this will get you started. There are some super cool guys on here that will get to your thread when they see it but this will get you started.
Assuming your at 1.5 with uboats
RSRD run silent run deep
Improved stock enviroment v3
Maxoptics imo its good mod
Now if you want to run uboats
add Operation Monson to that list with the patch and upgrade
If you want hard Pacific
TMO its upgrade and patch
Have not tried RFB real fleet battle so don't know.
I am no vet here but those are some good ones.
Go here for some good info
http://www.subsim.com/radioroom/showthread.php?t=155786