You'll obviously need to download the MODs and the JSGME to install/enable them. It is also helpful to install MODs for other games.
Link to JSGME
http://www.subsim.com/radioroom/down...o=file&id=3559
I can only strongly advise you to use the mods Trevally highlighted, it will improve the game extremely.
In addition to the ones he pointed out, i can only recommend:
SteelViking's Interior Mod V1.2 (This will make all the dials in the sub actually work and overhaul nearly everything in the ship (valves, doors, floor) Personally, the colors where too bright for me so after that, i installed
Dark_Interior_V1 to have working dials AND a gloomy feeling)
MCCD "More Crew Commands by Dialog" MOD (this will make you able to give the crew more commands (eg. charge batteries to LI or depth under kiel/weather forecast to navigator. The best thing is that you can give some commands through the Officer on the Tower, no need to climb down. It runs smoothly with the
Mighty Fine Crew MOD.)
Mighty Fine Less Annoying Footsteps 1.0 (self explaining, stock sound is like a ballet dancer on a gully)
A good strategy when deciding about MODs is to just sort them by download count. A MOD with 5000+ downloads is usually sweeter than one with 15.
Hope that helps
good hunting!
P.S. Look at me, 10 days in this forum and already talking like a pro
http://www.subsim.com/radioroom/down...o=file&id=1671 Mighty Fine Crew MOD
http://www.subsim.com/radioroom/down...o=file&id=1912 Steel Vikings Interior MOD
http://www.subsim.com/radioroom/down...o=file&id=1573 Mighty Fine Less Annoying Footsteps
http://www.subsim.com/radioroom/showthread.php?t=179344 Dark Interior V1