Nasus is a melee champion. His primary use is a tank, but he can also be a good jungler or a dps(demage per second) tank. I play Nasus as a tank or a dps tank, and so do lots of people. He is a great laning chapion, and one of the best 1v2 lane champion(if someone from you're team is jungling).

Nasus's passive ability is Soul Eater, wich gives Nasus's attacks 10/15/20% lifesteal.
His 1st ability is Siphoning Strike. This ability increases the demage of Nasus's next attack based on his attack demage. This ability is good for last striking.
2nd ability is Wither, where Nasus slows his target, decreasing is movement speed for 5 seconds. This ability is great for ganking, where u wanna slow you're target and make his runaway impossible.
3rd ability is Spirit Fire. Nasus creates a cicle of power, wich draines target's life every second. This ability is great when you're 1v1, or when u jump in team fight.
Finally, his ultymate is Fury of the Sands. Nasus grants a sand cloud aroud him, that grants him more health, but also drains health from enemies around him and turn's it into his demage.

If u play as a tank, then u should level Spirit Fire, but since i play as a dps tank u mainly level Siphoning Strike, and after that i level Wither and Spirit Fire. Of course, u level his ultymate at level 6,11 and 16.

For Nasus, i go for 0/21/9 masteries, wich grant him aditional armor, magic resistance, health dodge chance ap and dp, but also health and mana regen and increased neutral buff duratation.

For his runes, i go for armor penetration marks, armor seals, mana regen glyphs, and health quintessence. I close armor pen. marks because of his 1st ability, health quitessnece and armor seal for tanking, and mana regen glyphs because he lose's mana pretty fast.

As my starting item, i chose Doran's Sheild, because it grants u health,health regen and armor, wich is pretty good for tank like Nasus. My second item is Sheen, wich gives mana and ability power, thats is good for his 1st ability, and also gives u freedom 2 cast abilities without carying about you're mana. For boots, i chose for Berserker's Graves, that give u aditional attack speed, wich makes Nasus to attack a litlle bit faster, because without them, u attack like a 100 years old grany. If enemy team has a lot ap champs, u can easyly replace them with Mercuries Treads. My next item is Phage, that grants u health and demage, even more increasing you're 1st ability. After that i go for Aegis of Legion, that gives u health, magic resistance, and armor, wich is good for you're tanking. After that, go for Trinity force, cuz with all things from Phage and Sheen, u gain also critical chance and attack speed from Zeal. Next item is Warmog's armor, that grants u health regen and health. This item can also be replaced with Randuin's Omen, wich is also good cuz it grants armor, health and health regen. Finally, my last item is Hexdrinker,that gives you demage and magic resistance, both good for u

As summoner spells, i chose Clense and Fortify, 1st good for tanking, 2nd good when ure at the base and enemy is pushing lane without anyone to defend the lane.