Skarner in my opinion should be used as a speed snatch, grabbing and dragging the ap/support to your team for quick elimination. Also his constant slow gives him a almost support dps role.
Skarner is fairly weak early in the game, so until lvl 6 I suggest you AVOID any confrontation except for ganks and first blood.

First of all, this build is for a LANE SKARNER not for a JUNGLE ONE. In my opinion, Skarner is a slower jungler, with lower ganking capabilities in the early game, before lvl 6, but he is one hell of a laner, and can even go single on top, against 2 opposing champions, so it really doesn't pay to play him jungle.

This build makes Skarner a very fast scorpion. So much that, if he weren't a scorpion, he'd easily be confused with SpeedyGonzales, or RoadRunner. This build makes Skarner tanky in the lategame, but until you finish Shurelya, he will die if you dive in carelessly. He works better once the tank dove in the enemy team to get all the damage, or by ambush from a bush or out of the jungle. And always go in with the exoskelet skill for the shield and speed. You need it to grab the squishy and get him as far into your team as you can.
Once your ulti has been landed and you got the squishy in the middle use your Q to slow him and give that extra dmg thanx to Sheen. So spam it(your Q) like crazy!

Skarner's build is pretty much good all-around and in any situation, so except the last 2 items, you don't need to change anything depending on your current game conditions. Mostly, even with full farm and a bunch of kills you won't get past Shurelya too often, mostly because they're all expensive. But really, all Skarner really needs are boots, some mana regen and Trinity Force, so you can play around with your item choice after you get them, but I strongly suggest that you build him tanky afterwards.

I strongly suggest Flash so you can pop in the face of your opposing team and grab a squishy fast, that' s a good fight starter, as long as your team follows. Then you can either get exhaust or ghost, depending on your play style, and what the other have gotten.

When playing ranked, and you got a jungler on your team, go TOP. Skarner can solo against one or even two players, so it's a good idea to do it. And the team won't curse your early ulti too much. But even if you are solo, don't forget to go gank from time to time...
If you're up against two opposing players on top, be safe though, as Skarner is squishy in the early game.

Pros:
* Ulti is used at maxx capacity, so you can get a lot of focus targets down quick.
* Tanky in late game.
* All that speed increases Skarner's survivability, and even if slowed, he can make decent getaway.
* Good ganking capability, even higher than a jungler Skarner at some occasions.
* Speed improves the chance you get to slow your enemies.

Cons:
* Squishy(has a talent to get beat up)Late Early and Mid Game.
* The Core Items are kinda expensive, so you have to really farm early game. So much that you may even consider getting fracture first(your E skill) to farm better.

That's the most important part, and don't remember, when you play a caracter for the first time, do it with bots, or say that your first time, so your team-mates won't curse the hell out of you if you feed.