Okay so this is my first build on Mobafire. It's more of an idea on how to play Nocturne, wich I'll update over time. Right now I'm slightly on a rush, still having a few problems with it, so it's not very good. I hope you like it and it gives you an idea onto Nocturne. I'll also add more details into this when I get the time.

Thank you!

Main items:

Berserker's Greaves - Self explainable, movement speed and bonus attack speed.
Manamune - If you have a jungler in your team, or you have someone else who's really mana dependant, I recommend you get this item, as it gives you some bonus AD and some nice mana. I've noticed that you may run out of mana really quick in ganks/laning. This is a great item.
Youmuu's Ghostblade - Cooldown reduction, armour penetration, critical strike, attack damage, and an epic active. I've had second doubts about this item, but after I've tested it a few games, I've noticed it works really well on Nocturne.
Banshee's Veil - Bonus magic resistance, and tadaaa ...a second spell shield. It will increase your survivability really well.
Phantom Dancer - Attack Speed, Movement Speed and Critical Strike, really good item on Nocturne. You may get this early on in-game if you feel so.
Bloodthirster - Epic damage and life steal. Works really well with Nocturne's passive, and it gives you greater chances of survivability.

Alternatives? Yes, we have those aswell:

Zeke's Harbinger - Pretty good item on Nocturne if you want more life steal, and attack speed, depending on what you'll preffer to replace with it.
Executioner's Calling - I'd recommend this if you have a Swain or Vladimir in their team, that's creating you problems, as it will really reduce the healing they receive from their abilities. Also gives you some nice Critical Strike and Life Steal.
Infinity Edge - If you really want to go on the field crittin', then you may replace Bloodthirster with this item.
Trinity Force - I didn't had the chance to actually test it out on Nocturne, but I say it would work out pretty well if you want to play around.

What I use:

Exhaust - It really helps me a lot slow down those rushed enemies, also helps against other AS/AD champions like Master Yi or Tryndamere.
Flash - Good spell to catch an enemy, or flee from them. I found myself in a situation where it really helped me get a kill on Ezreal.

A few alternatives:

Ghost - Take this if you preffer the bonus Movement Speed it gives.
Ignite - If you really hate to see fleeing enemies, then this is for you.
Clairvoyance - I didn't tested it, but if an enemy manages to escape, you may combo this with your Paranoia and catch him. Watch a small guide here on how to use it. (not taking any credits on the video)
Teleport - If you don't like missing from your lane, then you might consider taking a teleport spell.