Skarner needs to farm quickly in early game so he unlocks impale for easily ganking. He kills every neutral monster pretty easily, without losing much health. When you go for a gank, use Crystal Exoskeleton to reach the enemy faster. Invading at the enemy jungle and stealing the blue buff, will give you much time to farm and gain lvls or gold.

While Ganking use Crystal Exoskeleton to reach the enemy faster. Then use Crystal Slash and waith for its cooldown, using Crystal Slash again will slow him, and you can pull him with Impale to your teammates to finish him off. Try to let kills to the carries, skarner doesn't need any.

Skarner can support every lane, and can easily gank from every lvl. Although he's a bit squishy at early game.
Skarner's Ultimate is the best way to secure a kill. However, Fracture is really useless at the begging.
Skarner can tank the game pretty well. But he can't save a low health teammate.