I use my runs to get both passive abilities at LV 1 dealing great damage with my initial 40 AP and my 10 Atackbonus, farming mobs more easily.
Depending on your enemies I play agressive, constantly spamming my seal on the enemy heros, or play def, hide in my shroud and kill minions with my seal or with last-hits.
At Lv 6 I should have my boots (MR) and my Hextech Revolver (additional a Elixir of Brilliance) and start playing agressively pushing to the enemy tower.
When they don't come out, you can change your lane and try to gank someone or steal the blue buff.
When you finish your Gunblade you should solo nearly everyone, healing you with your abilites, slowing with Cripple and your Gunblade Cast, dealing additional 300 dmg (which is a lot)

After that go buy your Sheen for more dmg. Finishing it to Lich Bane gives you more survivability against caster and a lot more damage.
I buy Rabadons first, because with my cripple and Gunblade i can slow enemies as good as with Rylais. Dealing more damage than the enemy you don't need ne 800 HP from Rylai's so you should be fine without it, for now.
After you buy the Rabadon your AP should match everyone else's or surpass theirs.
You get a lot of kills.
Don't start the teamfights and when you get focused, use your shroud and focus the enemy casters or squishi dps.

Never forget to use your Ulti against mobs to win distances and run away from a gank.
It will save you a lot of deaths and more ganking surprise when you appear near a hero with your ultimate.

If you have dificulties with teamfights, or get focused a lot of time, get Banshee's which gives you more survivability and a great Magic Shield.

With this Guide you should rock the mid lane as well, hiding and at LV 6 starting to push and gank easily on other lanes.

in Mid/Late Game start with this Combo:

Q, (wait till the Cooldown is gone), R, Autoatack, Q, W, R, and you should kill any enemy caster who doesn't have infinite HealthPoints.

While waiting till your Q Skill cooldowns, you regain a bit of Energy and you can cast it again, after going on the enemy hero with your ultimate R.

When you port back, getting the Red Buff is a good damage bonus and a great slow for Chases or Ganks.

My favorite Character in the game.
Very flexible, great chaser and ganker, lot of fun to play and pretty unpredictable.

Greetings t(`-´t)