Hello, this is Matt and is this is my first guide on Mobafire.
This guide will help you build a nocturne that can carry your team with some bulkiness as well.
If you disagree please read the entire guide. Also I am sorry for the lack of pictures as I do not know how to add them.
Please do not downrate the guide if you do not read the entire thing. If you do downvote it please give me a reason why and I will try to find an answer.

For marks i ussually take Greater mark of Desolation as Nocturne is an AA with a passive that works with attack speed. Also I take a few Greater marks of alacrity for a little AS boost.

For Seals I use Greater seals of Resilience to help in that early game stage where Nocturne is quite squishy. You can also substitute them with Greater seal of Vitality.

For glyphs I take per level magic resist as they will surpass the flat runes at level 8. But you can substitute with the flat runes.

21/0/9
The masteries set up is focused on you being able to kill quickly and effieciently. I reccomend taking Ruinic affinity just so that your red and blue buffs are more effective and can be used while ganking in mid game. Just make sure that if one of your teamates is more suited to taking the buffs. LET THEM HAVE THEM. You could take the 9 points in Utility and put them into Defense, for more early game endurance.

For items. I ussually will take boots of speed and 3 health pots for some sustainability in a lane. You can also take cloth armor if you are jungling but if youre laning then stick to boots. Nocturne himself is very squishy without any kind of item bonus so i will ussually rush an Infinity Edge. But if you find yourself always being killed get yourself a Thornmail for heavy AD teams and Banshees veil. I will always get these two items anyway since the Banshees viel gives you another spell shield on top of the one you already have. After you have those items Begin building a Frozen Mallet for a 100% slow buff, AD, and a good bit of health. All around good item. The last item, if you are fed and fat, i would recommend getting a bloodthirstir for the extra life steal and AD. Alternative items include, Warmogs Armor, Madreds bloodrazor, Hextech gunblade, Guardian angle, Phantom dancer, and Last whisper.

As your first skill always unlock unspeakable horror Lv 1 as you can fear the enemy in those few minutes in which you gank mid or top. Then unlock duskbringer so that you have a way to chase an enemy and you can also poke them with this skill as it has decent range. At Lv 4 unlock Shroud of darkness. The attack speed is nice and this gives you a spell shield to block that Rupture from cho'gath, and any other slow spell like Veigar's Dark Matter.
At level 6. Unlock Noctunres Ultimate. Paranoia has ridiculous range and is perfect for killing that teemo thats running away with 500 MS and you cant catch up to him. BOOM hes dead. Never initiate with this skill as it has a lengthy cooldown and its better to initiate with duskbringer for the AD and MS.

THE GOOD.
Flash - In my opinion this is the most versatile Summoner spell as it can be used to flash over walls and escape death. It can also be used to flash into a fight and get closer but, i usually dont have to with Nocturnes speed boost from Duskbringer.
Ignite - Good for getting that kill that may have gotten away with that 50 health. Also good since it reduces healing so pop it when your using Unspeakable horror in a teamfight so their carry cant heal themselves.
Exhaust - SLOW THEM DOWN AND MAKE THEM USELESS. Always a good spell.
OK
Ghost - Good for chasing but flash is better.
Heal - Good if your a beginner.
Smite - If youre jungling.
Teleport - Good if youre going solo top and dont want to lose a tower.
DON'T EVEN USE THEM. Ever
Cleanse - You have Shroud of Darkness and Banshees veil.
Revive - Long cooldown
Clarity - You wont need mana your AA based.
Promote - You arent a full tank. More of an offtank.

So basically all you have to do with this build. Is, stay behind your tank until you get close enough to lanch Duskbringer, close the gap with the MS bonus and then fear them with Unspeakable Horror. If you have your Frozen Mallet, then you will have them slowed to the point where they cant run away. And if the event they do get away (VERY UNLIKELY) Pop your ultimate and finish them off.

IF YOU WANT TO JUNGLE. Take smite and flash. intead of unlocking Unspeakable Horror get Duskbringer. I ussually go with Cloth armor and 5 health potions. Start at Wolves then to the Wraiths, smite the blue one. NEVER use smite on wolves as it will ruin your whole cycle. I ussually get Blue at around Lv 3 and gank at lv 4 after I have both red and blue. If you'd rather have a teammate leash you for blue go ahead. But, I dont like doing this as you need to go to the enemy teams blue and risk getting counter jungled.