Steel-types, to be honest, are probably the most game-changing, broken types in the game, only behind Dragon. Steel-types are sturdy little buggers; they are essentially Rock-types on some serious crack.
They almost always have high Defense or HP. Special Defense and Attack are sometimes prime features, and they seldomly have any form of usable Speed or Special Attack; they are Physically-inclined, 90% of the time.
Defensively, they are beasts; offensively, they're a joke. Steel-types have good coverage with their use of Ground-type and Rock-type moves, but their Steel-type STAB covers two measly types, both of which are mediocre or rarely seen, meaning Steel-type STAB is virtually useless.
Steel-types are made much better with a secondary typing such as Ground or Psychic, seeing as a weak STAB option leads to a weak offensive Threshold. Some popular Steel-types that prove this are Excadrill, Steelix, Metagross, and Bisharp.
I can't really tell you which types are useful, but try your best to use this information to come to your own conclusion!