last hit with siphoning stike(q) whenever possible as it gains 3 dmg for every unit it kills
use sprit fire to farm if your solo so you dont get attacked and have to recal back to base
using wither on a physical dps champion will reduce their damage output as it decreases movement speed and attack speed
try saving your ult for team fights because it can save your life if used at the right time

start at the regular golems and autoattack 1 then smite it once it will kill it
after completing that camp go to the wolves do Not use smite on them
after that camp go the wraiths and by now ur smite should be back so use smite on the blue wraith and autoattack/use siphoning stike to finsh the rest then go gank or go back to the regular golems then gank if u havn't already then go the golem with the mana buff/cooldown if u have a mage like annie,swain,etc give it to them then go gank again and after the gank go to the lizard camp and after finshing the camp gank
repeat this pattern until level 15 at level 15 start laning more/push turrets and kill enemy champions

tank nasus tends to spec more in the defnsivve tress
ad nasus goes almost completly in the offensive tree

start with vampric scepter this will keep you lane longer and very durable in 1vs1 or 2vs2 or in team fights after like 5 minutes buy boots of speed and some mana pots and hp pots after you run out of your potions if you have low mana or hp go back to base and buy mercrury treads or the dodge shoes it will make you survive more likely in team fights becaus of the doge chance or lower duration on disables
upgrade vampric scepter to wriggles lattern after you finsh wriggles lattern buy phantom dancer or trinty force either works but i prefer to get phantom dancer for the attack speed buy frozen mallet it'll make enmies think twice on focusing you with your ultimate because you will have over 2.5 k health buy a last whsiper for armour pen for those armor stackers like rammus and malphite get your infite edge to make your crits hurt and do like 350 or up to 400 damage and nasus's siphoning strike(q) can crit
sell your wriggles lattern and buy bloodthrister

heal-same as cleanse but if ur vsing an ad enemy team cause cleanse is useless no ad carries have a disable
smite-a must have if ur jungling with Nasus and virtually any jungler
teleport- if ur playing an agreesive Nasus this is okay i guess
surge- helps u destroy enemy turrets faster and helps u in 1vs1 fights with ur natural lifesteal
flash-helps u escape possible ganks especially when ur jungling
cleanse-often when playing ad/jungle Nasus u'll get spammed with disables but this fixes some of the problem
exhauts-helps u gank and strong in 1vs1/team fights because it reduces the targets damage output and item damage by a percentage

revive-what are u planning on trolling the whole game
clarvoyance-ur not a support need i say more