Feel free to comment and tell me on what things i did mistake, which ones you dont like and which you did =) it will help me write future guides since this is first one i write

I remember when i first bought nasus i made him full tank, but he couldnt do enough damage so, after checking a few guides here and playing a lot with bots i found, at least for me, the right balance between tank and AD on nasus =)

Pros:
Very high life and nice life steal
Nice farming skill
His slow its good if team has other Crowd control
Very flexible champ, he can either be tank, carry or offtank according to build
his ulti will make people cry =)
Funny dance =D

Cons:
Spirit fire damage its kinda low
Only Slow =(
Without a proper last hit with siphoning strike, his damage will be bad

Soul Eater: it gives life steal which works well at early mid and late game =) specially mid, if your dying all you need to do its use Siphoning strike and you will get life back

Siphoning strike: you first skill and by all means you primary source of damage, the damage can get higher as you last hit with this skill so do it =) it also works very well with sheen, since it will double the damage, you should level this to 3 asap so you wont have problems to last hit minions

Wither: you only crowd control ability, that slow and reduce enemy attack speed, good thing about this skill is that the effect gets higher each second target its under this spell.

Spirit fire: you farm ability =) this skill maxed can kill caster minions and leave melee ones to get hit by your siphoning strike. This skill can also check bushes, i like to max this first since farming means more gold and experience =) so you can level other skills faster

Fury of the sands: Your ulti and one of the most hated skills on leage of legends. with this ability you can drain enemy life, make higher damage and also get higher life, which its perfect since we use atma impaler =), dont use this to iniciate fight, wait until they focus you and let them suffer.

For items, considering this nasus doesnt have a good damage until late game i pick both philosopher and heart of gold, you can sell this two items after the 30 mins mark of game so it will pay themself =D. also philosopher can help with mana issues at early game, for late game you should ask teammates to let you pick blue.

Aegis of legion its a very very good items for either tank or Offtank, as it gives a good armor,magic armor,health and also has aura for you teammates, and its very very cheap items, so its a must =) late game you can sell this for either thornmail or sunfire cap.

Sheen works very well with you first skill, siphoning strike, and gives a little of mana which nasus needs badly.

Warmong and Atma impaler: this two items together will give you tons of life, and life will trigger Atma passive, which will make you hit really really hard =)

Force of Nature: this items goes mostly because you lack magic resist, and also it gives tons of health regen and its passive can heal you 15 health per second, i know it doesnt sound like much but it is =)

Trinity Force: this goes here because of its passive, its kinda like an upgraded sheen and also gives more movement speed

Also, as a sidenote, if you have some leftover cash after buying try to get wards, it can save you for getting ganked for a jungles or u can teleport to them and help an ally or help gank by moving behind enemy tower

For shoes i have 2 choices really, first one its ninja tabi, for a heavy ad team, also because it will trigger masteries =), for other choice i pick mercury treads, for a heavy ap and stun team. you can also feel free to pick boots of swiftness if you need to be a bit faster.
Personally i prefer to pick ninja and later change for mercury, because at early game not many champs can do high ap dmg unless they are feed =)

On this topic i dont know really what to use since i didnt got full runes set yet =) but i guess usual hp/magic resist should work for nasus

i like to pick teleport and ghost for two reasons:

first one teleport can be a tower saver =) you can teleport, use your spirit fire on minions or wither+ulti if enemy champ its there.

ghost its good to chase enemies that run away after team fight failed. or also if you wanna kill a squishy champ like evelynn or twitch

Sometimes i also pick fortify if no one in my team choosed it because at early game a good use for it its kill enemies near towers, and at late game it can give a few seconds if allies are all dead and you need to make time until respawn

Other spells that can also work are exhaust or ignite, try to speak to your team to see which one fit better

Nasus can solo top if you got jungle or he can push really hard if he has a nice ally =) specially if he has a ranged ally, that way he can siphon minions while ur ally harrash enemy team =)

Mostly all charas are good team since nasus its very flexible =) but he shouldnt team with other tank because they will be very slow to kill minions and champs, but there are some exceptions, alistar its a deadly tank ^^

Good allies for nasus (personal favs ^^):
Sona (her stun, healing, and auras are very good)
Nidalee (his slow and her spears are a pain + very nice healing)
Fiddlestick (his fear and ulti stack very well with nasus ulti)
Udyr (very annoying for enemy team at jungle, he can jump and help you kill)

Bad allies for nasus:
Sivir (she wont let you farm siphon since she is a minions killer)
Veigar (same as with sivir, he needs minions more than you)
Mundo (this is personal one, i hate mundo)
Taric (he lack of hard damage skill)

Also try to get dragon if no one on your team its doing it, dragon its very important for extra gold

Usually this kind of fight happen after the 10-15 min mark, when whole team try to get a tower.
Your job in this cases will be to tank and slow, if your whole team its close to you, run right into enemy team and ulti, while using wither to slow support or carriers and siphon to help on damage dealing.

If allies are too far away you can either hold tower using spirit fire and ulti when enemies get close or run away to back tower =) dont be afraid to run away, 150g for tower its better than 300g because of dying.

[*] Always try to get both philosoper and heart of gold by the 8 min mark, so when you sell you will recover gold and even get a little spare =)
[*] Try to learn how to last hit with siphoning strike since that will means the difference between a low hit nasus and a high hit nasus
[*] Learn when to use you ulti and when to save it, lets just say your solo agaisnt lux, using ulti wont help since she can snare you and you will lose a few precious second of ulti, but if your facing a tryn feel free to use it =) he can slow but also you can
[*] Also with ulti try to use it at team fights, i know before i said if they dont focus dont use but, if they are all attacking other ally use it =) your ulti can drain enemy life really fast and, with siphoning strike and spirit fire you can make them escape and leave your ally on peace
[*] If at start of game enemy has a jungle, why dont you surprise him with rest of team? it can mean you will get first blood =)
[*] Always have fun on games =) even if they dont start well mostly they end good