I play Nasus pretty weirdly since I'm good at keeping him out of a fight till level six and about 50 q dmg.
I typically run 4/14/12 (utility down to the gold per)(offence for cdr)(rest for tanky masteries) with flat ad reds, armor yellows, mr per level blues, and either gold per/movespeed/flat ad quints.
I generally grab either grab 2 wards 9 health pots/longsword 2 pots/rejuve bead faerie charm then pots for my starting items.
If I need to leash I tank the buff for my jungler then go top and farm Q
I level q then w then q then e for an early advantage on my q cooldown and base damage. Normally I last hit the wave until I am last hitting under tower. Normally I farm to about 100-150 q damage (or get zoned out of lane) until I back.
First back I normally grab philostone and sheen if I can.
Then you should be able to one hit the caster creeps and continue farming Q.
Max Q first then W. I max Q because I'm Nasus and you ALWAYS max Q unless you're trolling with ap nasus. Then I max W because it's your chase and it slows their attack speed.
My final build is normally: Merc Treads, Triforce, (elise's miracle passive), Spirit Visage, Warmogs, Manamune, and Bloodthirster.
My more tanky build is: Merc Treads, Triforce, (elise's miracle passive), Spirit Visage, Warmogs, Runic Bulwark, and Frozen Heart.
My full tank build: Mercs, Iceborn, (elise's miracle passive), Visage, Warmogs, Runic, and Randuin's.