you want to do, blue-wolves-witches (recall) red-golems once you have done this you can follow around this anywhere. you would also want to do dragon asap.
skarner can become useless with no mana, so try and keep a blue as much as you can, if low then dont spam q.

I build into speed as early as I can afford, this is so you can catch and drag anybody with your ultimate. also as you build a tanky build, you also build an atmas. this increases you damage output, as well as giving you armour. if you were against a mostly ap enemy team, you would want to build into more of an magic resist build for example getting a force of nature or banshees veil. if you were not getting targeted then you would also want to build into damage then into the first 6 items rather than the guardian angel.

your ganks arnt to good until lvl 6 so its best to try and rush to lvl asap, then gank...alot. however if you see an opportunity, then gank. when ganking, spam your q for damage and landing two close together q's you slow the enemy.

your role can be a tank, however if you get fed, you can become a great damage dealer, your q will deal a large amount at lvl 1 and can do lots of damage to enemy champions. this is why they will target you, so you need to build quite tanky.