I am a level 30 LoL player in the NA Server (IEatphresh) giving you a guide on nasus . I think i am a good nasus and this is how it got me out of bronze to GOLD .

I choose for marks Armor penetration because , armor pen is good at top lane because you can tear their health down with your Q and you dont need the extra AD .
For Glyphs i chose magic resist ones because most champs have some AP damage in their skills
Seals i chose armor because you will get often harrassed at top as nasus and you will want the armor
Finally i chose Life steal for quints because your passive gives you some life steal and when you last hit with your Q , your opponent depending on who they are will poke and harrass you and you will lose health , so if you last hit with your Q you will gain alot of you HP back to fight when your Q stacks are up .

Mercury's TreadsI choose The magic resist boots because you need the tenacity so when you chase as nasus and they try to turn and stun you wont be stunned as long and can wither them

Trinity Force > Iceborn GauntletOne reason why i choose trinity force over iceborned gauntlet is because , nasus has enough slows on his wither (W) and so he should focus a bit more on damage , since all of his other items on my build are based of off tank .

Randuin's OmenRanduins is good on nasus because you can slow in teamfights and you can slow as you are puruing the enenmy

Locket of the Iron SolariLocket is good because , in team fights you can make plays as to saving someone about to die and it is an extra shield to shield your teammates

Frozen HeartFrozen heart i like on nasus i because , it gives me some good CDR , (cool down reduction) and you need mana on nasus to farm and slow enemys running or farming cs with your Q

Spirit VisageFinally Spirit visage because , it works well with your life steal passive, and gives you some great magic resist and CDR.

Good Summoner Spells for

Nasus
I choose Teleport because , when you base and you want to get items , as Nasus you dont want to miss CS so you get Tele so you wont miss any farm
I get ghost over flash because when you chase an enemy, you have your wither to slow and so you can chase with ghost if you aren't fast enough , and if you are trying to get away it is more efficient than flash because when you run away you run fast as well as your wither to slow them to get away.
Ignite isn't the best choice but isn't the worst choice , if you cant farm well with your Q, you can just fight and if you cant kill them , you may rely on your ignite

Bad summoner spells for Nasus

barrier isn't that well on nasus early game or ever because , if you have life steal quints and your passive and or spirit visage , you will have enough life steal to sustain in lane and if you can cs your Q well , you wont need to rely on barrier to save yourself unless you are ganked but not neccessary.
People say that revive is good with tele on nasus but honestly , its a waste of a summoner spell even if you are losing lane , you can handle just 10 cs or a few seconds of waiting early game , and if your team is relying on you late game to save it i guess revive would work but it doesnt help you early game as much as ghost would .
Flash isn't as efficient as ghost because at top or jungle , when you gank ,flash can't help you catch the enemy as well as ghost does in ganks or in general. If you are trying to get away flash can save you in some cases when they have no teleport or flash up, but if they do you die , as if you had ghost , you are probably tanky and can slow and run fast and get away with a higher percentage of surviving

I choose to max Q first because it is your main source of damage on nasus , you shouldn't build damage on nasus. You could but i choose not to because , as nasus when you farm your Q, you do massive damage and you can be tanky and sustain in fights longer and get away alive .you do alot of damage and be tanky. If you buildblood thirster damage you do alot but will die as soon as you get into a fight.

<--- (wither) I max out Wither next because it slows alot and will help you chase down enemys with your team to pursue someone while ganking or being helped counter gank someone

I max out spirit fire last honestly because you dont really need it at lane , because you should focus on farming your Q to do massive damages .

Obviously i max this out last because its the last thing you can max out cx.

Farming as Nasus is VERY important!
Farming is the key to Nasus being OP , his Q is his main source of damage , when you last hit a minion with his Siphoning strike , you gain permanent 3 damage on that skill . So if you get lets say 100 creeps with last hitting with his Q , you get 300 permanent damage . When you last hit a seige minion or a boss jungle monster , it gives you automatic 6 permanent damage (6 stacks)
Creeps and farming are like Nasuses survival of damages

PROS
- He is a great tank and can slow enemies while pursuing them .
- Massive damage with Q and still be tanky
- Chunks towers and can push well.
- If underfed , you can farm your Q and still do alot .

Cons
-If you dont have any cs you cant do anything as nasus
-Cannot do much in a team fight except tank and slow
-Without being tanky not very strong

Overall Nasus is a great champion , he has a great slow that you can use to help chase and pursue enemies trying to get away. He has a strong potential of tanking and doing massive damage. All you do as Nasus is build tanky and farm his Siphoning strike because its his main source of damage. He is one of my favorite champions because he is a quite simple champion and you just need to know how to farm and build as nasus or i have provided you a guide up above to help . And uh vote on my builds and ill post more thanks im starting to make new ones