For Masteries, you should start with offensive. For a build, I think you should start with boots and 3 HP potions or 1 ward + 1 potions so that you can place it in the brush at the river. :) That way you won't be ganked.
Then from there try to take every creep so that you collect the money quickly. Then it depends with what hero you are in lane 1v1. ;)
Do Wrigles lantern. It's nice because it raises your stats and you get 75 gold for wards. ;) Then you shoud do Atack speed - Damage and Lifesteal items - for fast kills.
For an assassin like Nocturn, it's nice to make Frozen mallet for the slow effect. From there you should look at what type of Resists you need. If its Armor you need, you do Armor items or little more HP (Health Points). If you die fast from Ability power, do little magic resists. But if you don't push yourself, you will never find what you need. :)
Do many games with Nocturn and you will learn my friend :)