Well this is how I play Nocturne: Always, always, always try to get first blood. The quicker you get sword of the occult the faster your damage increases through kills\assists. Basically, like every other champion try to get last hits on everything.
http://www.mobafire.com/league-of-legends/edit-build#

I go pretty much full armor pen, crit, cool down reduction.

Check masteries build

The reason I choose 2 bloodthirsters is because in this build you survive through your damage. Go in to a team battle and cleave the weaker champions, with around 50% life steal and hitting loads of damage you can live through 3-4 people whlist your team is also picking off targets. If your team needs a tank you can substitute a bloodthirster with a warmogs and you'll have roughly 3k health and lose only a bit of damage and lifesteal. I like getting warmogs if im playing a bursty team even if we have a tank.

If there are no team mates or others are recalling and you see large minion waves, get there fast as you can and duskbringer the mob and just cleave it down. Remember the faster you get this build, the higher your damage is and you'll almost be unstoppable.

Check skills.

I prefer ignite and flash for basically 2 reasons, damage and survivability.
The ignite really helps in the early-mid game by possibly kill stealing (more for you!) and the flash is great throughout the entire game. You can paranoia in a team fight, pick off someone and flash behind a bush and then come back in and pick another off.

Basically you need to get kills fast and be there for all team fights to help build up sword of the occult. If possible always get the neutral monster buffs. Getting first blood is key to a quick build to stay on top in damage.