This is my first build but let's get to the point:
Nocturne is an amazing character, he has crazy attack speed and awesome abilities, but first let's disscuss pros and cons
pros-
awesome ulti
fast attack
awesome attack damage
great low level
good chaser
cons-
squishy mid and late gamge
Hard to take tanks late game

I chose Exhast and teleport because teleport is awesome for carrys and that's what you're going to be, I also chose exhast because its just great for assasins period, you can pick many various spells such as ignite and flash but let's look at the ones you really shouldn't be using
Rally- Not good
Clairvoyance- I don't see why you would need this
Revive- if you even consider this go play evelynn

I went with an AP/AD build because nocturne's moves are good, straight up, top that with some attack and you have yourself a killing spree. The gunblade and the rageblade are perfect for this.
If you have troubles with tanks substitute lich or rageblade for Bloodrazor depending how much you need it.
Attackspeed is needed too much here with your W giveing you 40% and maxed.

Nocturne has extreamly unique moves, especially his ulti.
I like to start off with R or finish someone off with R if it's up, Then I go for Q and right after E, by then you got in 1 or 2 auto attacks and they are running away. Here's where E kicks in. If you did it right Q should have made a shadow trail follow him letting you stay in the range for E and fearing him. While he's feared your Q and E and most likely on CD so auto attack the **** outta him, then try to finish off with another Q and E combo. Also there has been alot of confusion with Nocturne's R, basicly just hit r and click on the enemy to jump to him.

Nocturne is OP and playing him is easy, he will make your enemys QQ at his OPness (haha >>) and yeah kthnxbai