Nasus is a top lane champion that can be utilised in more than one way. He can be an offensive bruiser, an un-killable tank or a solid bodyguard. His abilities w, e and r scale very well into late game and his q can scale indefinitely. His main weakness is his poor early game and laning phase.

The most important thing about Nasus is his q ability. It is an empowered auto-attack that gains damage when he kills a unit with it. Small minions grant 3 stacks (+3 damage), while large minions, monsters and champions give +6 stacks. This ability is what makes Nasus what he is, and he is a champion that scales extremely well into late game and can even end up carrying.

Summoner spells are the two abilities that are selected in champ select before the game, and I prefer to take teleport and flash. Another summoner spell that works extremely well is ghost, and ghost is generally more popular. However, ghost is better for late game, where Nasus already excels. It is supposed to provide extra mobility, and prevent things like kiting. In my opinion, Nasus is strong enough to survive without it late game, and works better with flash, which is much better for laning phase and increases chances of survival and early kills.

Nasus's six core items are Mercury's Treads, Triforce, Frozen Heart, Spirit Visage, Sunfire Cape and Randuin's Omen. These items provide a good balance of damage, health, armour, magic resist, movement speed, tenacity and crowd control.
Situational items:
Banshee's Veil - Can be swapped for Randuin's Omen if the enemy team has 3 or more Ability Power based champions, or one that is getting fed and out of hand.
Thornmail - Can be swapped for Spirit visage if the enemy team has 3 or more attack damage auto-attack reliant champions, or one that is getting fed and out of hand. However, this will not be very likely as Frozen Heart, Sunfire Cape and Randuin's Omen usually provide more than enough armour.
Iceborn Gauntlet - Can be swapped for Triforce for extra mana, armour and crowd control, but at the cost of a little hp and damage.
Ninja Tabi - Can be swapped for Mercury's Treads if the enemy team has excessive attack damage or a lack of crowd control.
Last whisper - Can be swapped for Randuin's Omen if the enemy team is stacking armour and you really aren't doing enough damage. However, this won't be a problem most of the time if you have stacked your q properly, as well as the fact that your e shreds armour.

Nasus is a very durable tank with high stats, poor early game but amazing late game scaling, good crowd control on on target, high damage and can carry a game in many different ways.
I hope you enjoyed the guide, and find it useful.