I mained Nasus for a while he is insane, you wanna have flat armour pen red, armour yellow and mr blue runes and life steal quints. His passive alone is Op...having 14% lifesteal straight away is very good for lanning.
you defo wanna just stick up top and farm like a beast with your q, in terms of build it depends on the game....i normally build warmogs or sunfire straight of the bat....before tier 2 boots then if you wanna get offensive items go for it.
But in general he is a very skilled champ and you need paitenced to know when to att, i wait till around 4 to start going aggresive with 2 lvls on q 1 on w and 1 on e.
Oh and for summoner spells have ignite and exhaust....with ur w and exhaust its a easy kill, and if you get ganked you exhaust one and w the other hope this helped.