OK I want to say this right at the beginning. the reason i am making this guide is so that I will never again have to see a nocturn build incorrectly. So here we go! Hi everyone my name is badasabot and i have been playing league for quite a long time now and maining noc for most of it! why because he is one of the most versatile and powerful junglers in the game. For those of you that read these guid's often I would pretty much say the same things so I'm really not going to wast anymore time on this.
1 more thing I value the information on this guide not the way it looks so sorry for people that complain about lack of pictures and if you want to see good nocturns in action just go on youtube

Pro's
good clear speed
snowballs well for a jungle
versatile builds
has a spell shield (nothing feels better then watching the darius or garen ult fail miserably:))
most powerful ganking ult in game

Con's
short range on first level of ult
delayed cc
relatively expensive build
you are a jungler and everyone always hates on you for their feeding

I put my favorite runes in the above builds however because he is so versatile there are many different runes that work some of them are just situational.
possible quints:
Movement speed, lifesteal, armor pen
other quints that are also acceptable but nat quite as good are flat AD, attack speed quints, and flat CD quints to decrease the time between ults/kills

Marks

easier choice here armor pen, AD or AS marks are pretty much your only viable options

Seals

if you are in the jungle there is no choice take armor seals

Glyphs

For glyphs I have found that either CD or scaling MR works the best

My masteries are very basic I either go 21-9-0 if i think im going to be one of our high dmg dealers or 9-21-0 if I am going more tanky
the reason that I don't bother with the utility tree is that the only thing that would be really nice is runic affinity and it isn't worth 9 points just for that. also you should really be giving the blue buff to your mid and since you are building spirit of the lizard elder you don't really need red buff, its nice but not necessary

your skill sequence is very simple because noc if very auto-attack reliant. if you are using your full combo jump on them with R hit them with duskbringer your Q pop your E on the champ you are trying to kill and start auto attacking. also don't forget your spell shield try and save it for their strongest cc or a high dmg spell like leona's ult or Darius's dunk. if you are facing an enemy with little cc like ez then it might be best to just use it to block the first skill shoot because of the added attack speed it will give you from a successful shield.
one last tip about ganking, if you can just walk up the the person you are trying to gank do that because your R is your only gap closer and it is on a fairly long cool down so unless you need it to get in there you should save it for if they flash away or just keep it for some other gank

In the 2 builds above I already gave pretty much every item that you will ever need in most games as noc, but as always feel free to experiment and find out what works for you.