OK! heres a little to get ya started.
The only real key to victory in this game is: AP management, if you get the most out of every AP you will be in the hunt for the top spot.
Heres a little more for ya!
Shielders are key for defendeing your army
Fighters are key for attacking and for dying in defence
Torpedos are a must have (in huge numbers) at the top levels
Mine Grids are handy but not nescarry, they cost a lot, but if your rich they can take the edge off someones huge torp assault.