first off just start with my GFO mod
found here:
http://www.subsim.com/radioroom/showthread.php?t=151796
it will fix many issues with the game and add a lot of extra game content without altering the game or the difficulty levels.
beyond that, take a look at my list of my mods
found here:
http://www.subsim.com/radioroom/showthread.php?t=139472
the vast majority of them are designed to make small but important changes to the game to help your enjoyment but read each mod thread to see what it does for you.
after you get good at the game and are ready for a challenge then read the threads on manual targeting and start looking into the big mods like TMO, RFB, and FOTRS, they are all mods that make many changes and alter the difficulty of game play and attempts to make things more realistic but in their own different ways. each has its own flavor so when you are ready, you should try them all and see what you like best. many people play both RFB and TMO on separate installs.
FOTRS is a mod that you use by itself without other mods because you cant use other mods with it unless specially made to do so. this makes it not as popular but its still a great mod worth using, you just have to play it as-is.