Introduction
Nasus is very specific champion. Only Nasus and Sion get extra statistic for kill the creps (and Chogath - but he is less time, because he high coldown reduction ultima).
In my opinion nasus is one of the best champion on top and the strongest champion in late game. Of course he was weaknesses- he is the worst champion in early game and really sucks if enemy stop him farm (it is very easy, because Nasus is touch champion. When is like siege tower - resistant, slow and necessary if you want destroy enemy castle).
So is all on start, what you must now, if you not want be first blood ;p
Go to the "Strategy" if wish know how play and win.
Stats
Damage 53.3 (+3.5 / per level)
Health 410 (+90 / per level)
Mana 200 (+45 / per level)
Move Speed 325
Armor 15 (+3.5 / per level)
Spell Block 30 (+1.25 / per level)
Health Regen 1.5 (+0.18 / per level)
Mana Regen 1.32 (+0.1 / per level)
Abilities
Siphoning Strike - Nasus strikes his foe, dealing damage and increasing the power of his future Siphoning Strikes if he slays his target.
Active: Nasus's next basic attack will deal 30/50/70/90/110 (+) (+ ) physical damage.
Passive: Siphoning Strike permanently gains 3 damage if it kills an enemy unit.
Cost
20/25/30/35/40 Mana
Range
300
Wither - Nasus ages his target, decelerating their Movement and Attack Speeds over time.
Nasus ages his target over 5 seconds, initially slowing their Movement and Attack Speeds by 35% and increasing to 47/59/71/83/95% over the duration.
Cost
80 Mana
Range
700
Spirit Fire - Nasus unleashes a spirit flame at a location, dealing damage and reducing the Armor of enemies who stand on it.
Nasus unleashes a spirit flame at a target location, dealing an initial 55/95/135/175/215 (+0.6) magic damage.
For the next 5 seconds, enemies in the area have their Armor reduced by 20/25/30/35/40 and are dealt an additional 11/19/27/35/43 (+0.12) magic damage each second.
Cost
70/85/100/115/130 Mana
Range
650
Fury of the Sands - Nasus unleashes a mighty sandstorm that batters nearby enemies. While the storm rages, he gains additional Health and drains nearby enemies' max Health and converts it into bonus damage for the duration of the skill.
Nasus becomes empowered in the sandstorm for 15 seconds, gaining 300/450/600 Health. While the storm rages, he deals 3/4/5 (+0.01)% of nearby enemies' maximum Health each second (240 damage max per second) as magic damage and converts it into bonus Attack Damage.
Cost
150 Mana
Range
20
Soul Eater - Nasus drains his foe's spiritual energy, giving him bonus Lifesteal.
Strategy
On start you have good health regeneration and life steal - it should be enough to stay alive.
Because you damage isn't so much (and your opponent probably have much more), you must keep distans and focus on last it minions.
Enemy jungler probably visit you, so don't go to far away from your tower.
When your alliance come gank, use Wrie, Spirit Fire, Siphoning Strike, Exaust, Siphoning Strike, Write and he should die.
Runes
9x
Greater Mark of Desolation + 3x
Greater Quintessence of Desolation is the best choice, because your main abilities is Q (Siphoning Strike). It increase your damage = more steal healt = better survival and easier killing and farming.
Optional runes
For player who know, that p.damage is needed for balance damage team (sometimes you get ap team - in this situation phisical damage is welcome), so you can get
9x Greater Nark of Malice
+
3x Greater Quintessnece of Malice 
Philosopher Stone
1. Upgrade to
2. You can sell it on the end and submit some optional item.
Farming
Q is your the best skill and the strongest weapon. When you use Q for last hit, you get stronger, stronger, stronger and ... enemy accept surround vote ;)
Max bonus (+3 for kill minion) that i get in ranked game is +600 free dmg ;) and then ... game is not funny for five players in enemy team
One hit = one kill - that dream can be real, if you wisely farm Q (just that I wish You).
REMEMBER: coldown reduction greatly facilitate the farming Q. This is reason, why I always choose it. Because Nasus can play without blue buff, save it for your mage, that much more need it.
Skill Sequence
When your alliance come gank, use Wrie, Spirit Fire, Siphoning Strike, Exaust, Siphoning Strike, Write and he should die (you should turn on Fury of the Sands when you are near enemy - it probably will be before first use Siphoning Strike). Of course we don't use ultima if it isn't needed - it will be very stupid if you use ulitma for one enemy before teamfight (is one exception - if you kill enemy leader and other team suck, you can use all what you have).
In teamfight choice carry (range should be first target for your Write).
Siphoning Strike is your the best ability, so use all time.
Spirit Fire is dealing damage and showing enemy in bush.
Fury of the Sands can save you, if don't use it before and you run with ignite and die on moment. It is too good trap for noobs, which think that on moment kill you near your tower. You can say "Suprise!" turn on ulima, use Q (life steal restore health) and you get free kill.
HATE
List champions that Nasus hate:
1. Irelia
2. Garen
3. Gangplank
4. Tryndamere
5. Gragas
5. All junglers who want kill you.
5. Others champions that stop your farm and don't want be free kill ;p