There are many mods. It depends in part on how skilled you are as a captain. By way of examples, are you playing at full realism with map contacts off? Do you know how to attack use manual targeting? Do you know how to activate mods using JGSME? Do you know how to using the compass tool and plot a target's course? And if you are really nuts, do you want to learn to navigate using the stars? Many questions can be answered using the search feature, as this and many other posts deal cover answers to your question.
For starters, download the 3,000 meter bearing tool. If you do not want to load a supermod, then visit Webster's page and he has lots of individual mod fixes to the vanilla game (i.e. SH4 1.5.) If you do not know how to attack, see the Skipper's Bag of Tricks sticky in this forum. There are lots of turtorials there.
A list of mods can be found in the Mod forum under Neal Steven's sticky. It is not complete, but is a good start. Other mods are listed in the Subsim Download section, a link to which can be found in that sticky or contained in threads in the mod forum.
Welocme aboard, captain.
