Hello,
this is my Nocturne guide.

Early game:
Try always to deal damage with ur Q skill.
After your enemys are under half hp u can focus killing them with your teammate.
at 10 mins u will have atleast 1 kill or assist and you can buy The Brutaliser.

Mid game:
You should have Youmuu's Ghostblade and Beserker Greaves and If you're doing good you almost have Frozen Mallet.
Always look if there are enemy champions with low health so you can kill them with your ultimate.

Late game:
you should have your frozen mallet and a beginning of your infinity edge.
Just do the same as in mid game and try to kill their towers and watch your health in teamfights.