Hello, and welcome to my first ever build on Mobafire. I will be talking about my play style with Nocturne. This my not be the best build, but it's worth a shot. :)

Going an easy 21/0/9 build for agressive game play. you need the extra experience and gold for the whole game or you will get left behind by everyone else. The rest is for getting DPS maximized, which really matters for you to get the kills in 1v1 fights, which you should stick to.

9 Seal of Fortitude
9 Mark of Desolation
9 Glyph of Alacrity
3 Quintessence of Desolation

A great setup of runes for just the occasion of staying in a lane, and being the immovable object. A great amount of Attack Speed, and Armor Penetration will be great against those tanks.
The Seal of Fortitude is just a bonus, occasionally you would get more Attack Power, or Armor Penetration, but To stay in a lane for a long period of time you will need a reasonable amount of health.

You can start to harrass at lvl 2, when you have 1 point in Duskbringer and Unspeakable Horror. Start with Q, run to your target, use E and auto attack like there is no tomorrow. Use Q again when available. But never miss it! If you'll miss your first Q, the whole combo goes to waste. If you have problems with skill shots, practise vs bots or your friends on custom games.
At lvl 4 combo looks like this: Q, W (to prevent any disables from enemy), E, auto attacks, Q and so on.
Enemies will be like "OMG I just used my stun, WTF?! OMG he is hitting even faster now, someone help me, please! :("

And now... yes... this is what you were waiting for... lvl 6 and Paranoia !
Find a good prey for yourself. Use your ult, then use your W (yes, before using R one more time!!), then dash to your target, Q, E, auto attacks, Q and KABOOM, KILLING SPREE!
And why W before dashing to your target? If enemy is wise enough, he will prepare his disable for you.
For example: enemy Veigar is pushing top lane. You wait in the brush to jump on him, use Paranoia and dash on him. OMG what happened?! Oh ****, Veigar used his stun before you reached him... damn it... And he used Flash to get away...
I forgot this several times and trust me... it can end up even worse than just with not having a kill...