What you need to know:

Skarner is not a carry; he does really good sustained damage but isn't going to get you the massive crits.

Skarner requires a patient player. Someone overly agressive is just going to feed and make you look bad.

He is based heavily on items, and thus needs someone good at taking last shots on minions.

And he's borderline useless at early game because you're slow. Playing him isn't easy.

However when played correctly Skarner can dish out major damage, provide to be useful for the team, and take a beating.

So... if you read on I'll consider you to have the...

CHALLENGE ACCEPTED !

Source : Skead

Trinity force:

First and foremost; get this!
Starting with Shen, to Zeal,etc. This is a Skarner item.

Ninja tabi:

You may be thinking: "wtf, ninja tabi? Who uses that?"
It's for melee uses and towers 20% dodges suckas !

Hextech Gunblade:

Keynote: This is the FINAL item you should only get the Vamp gun and finish it later

Dancer:

Attack speed, critical chance, and move? Oh my yes!

Abyssal scepter:

Spell Pen, Ability power, and Magic resist. Get this.

Nashor:

Topping itoff with an attack speed, mana per 5, ability power, and cooldown reduction sword.
Excellent.

In my most utmost humble opinion this should be your primary choice.
Reason being is flash and ghost are VERY good offensive AND defensive abilities.
Keep it versatile.

Use ghost primarily to chase and drag. When you have Shield and Ghost popped you're able to grab
an enemy from the front lines and drag to the back with ease... and if you dodge an attack... it's even faster.

Exhaust - Very specific use; only good for chasing plus you already slow

Cleanse - Only useful for defense

Heal/Fortify/etc - Really not worth it.

PROS:

- One build that provides excellent sustained damage, survivability, utility, and speed!
- Works really well with most other players, especially melee.
- Can chain slow and run fast to chase.
- Due to attack speed your lifesteal is amazing.

CONS:

-Difficult to play, must be alert.
- Early game Skarner is weaker and very mana inefficient.
- Doesn't shine well in solo lanes.
-Can be harassed early game if you're agressive.