Hey this is my first build, its about nasus. I hope it will help u!

Why Ignite ?

It helps you finishing people early in game. Late game you can start ignite him to do some extra damage.

Why Flash ?

Nasus dont have an escape skill and can only slow 1 champion at the time, so flash is usefull to escape dangerous fights.

Other usefull spells:

Ghost- you can replace it for flash becouse you can escape with ghost to, but i prefer taking flash.
Clarity- Helps you farm minions quicker early in game becouse nasus has a low mana pool in early game.

Spells NOT to use!!!

exhaust- NEVER TAKE IT BECOUSE U CAN ALREADY SLOW WITH HIS W!!!!
heal- nasus his passive is getting increased life steal so u dont need it much, but if u think u need it u can take it ;)

Why those items?

Dorans shield--> Gives you some extra health and armor

Ninja Tabi--> Boots that give u armor, very good for nasus.

Trinity force--> will give you attack damage, attack speed, mana, health. Those are very helpfull.

Phantom dancer--> Gives u great attack speed and critical change!

Guardian angel--> for armor and an awesome passive :D

Warmog's armor--> HEALTH :D :D :D

Frozen mallet--> MORE HEALTH and attack damage + you slow them when u hit them, also with q :D

It's very important to farm minions with ur q!

when u farm minions with ur q u will do massive damage late game.

Some usefull things to know:
- Farm minions with q in ur e, ur q will hit more!
- Try early game to last hit minions with ur q