Use Siphoning Strike to last hit minions for the bonus damage, and it's VERY important for Nasus, it's like he's main spell, and you only need 1 rank on it in early games because you only deal 30 extra damage each rank, and it's like 10 last hits with this, so aim to rank Spirit Fire ASAP.
You can also use Siphoning Strike to nuke other champions if needed, but it's rarely you go 1vs1 in early games, so stay away from that, otherwise you can ask your teammates to gank your enemy if you're having problems. And I'd prefer you to not go solo lane, unless you're meeting an other melee champion because then you'd probably keep the lane pushed up to your enemies tower. To hold enemies back you can use Spirit Fire, and it will make them think, "Oh ****!!! This fire is killing me, I better stay back." But still I prefer you to not go alone.
Because you need stacks on your Siphoning Strike for late games, and if you go solo they'd probably harass you when you're trying to kill the minion for stacks to your Siphoning Strike, and that'll lead you to death.
However this champion is a great carry, so that's why I prefer you to buy Tiamat, and it's very good for you because your passive gives you lifesteal (Which is awesome) that'll keep you on your lane.
And your W Spell, is a great slow, and you should not use it until your enemy is fleeing!
Because I've seen people using it in early battles and it's just wasting mana and cooldown if used that way, first of all in battles, put a Spirit Fire under your enemies and then press your Q button (Siphoning Strike) and your enemy will take a effective burst (Because Spirit Fire Reduces your enemies armor by 20/25/30/35/40 which is a LOT! it's like 30% extra damage dealt). When your enemy is starting to flee you can use it and you'll surely get a kill or assist.
Some might think that Nasus is very squishy if played as AD but it's not that true, with his ultimate ability which gives 600 max HP makes you less squishy :D (Plus your lifesteal).
However in team fights, Tiamat and your ultimate and Spirit Fire deals a very high AoE damage and will probably make your team win in teamfights.
And you should use your ultimate in the beginning of every teamfight.
In late games you should still stack your Siphoning Strike ASAP (Because of some people often forget it).
You can also use Wither (The W ability) to flee.
Trinity Force will slow your enemy sometimes but if that's not enough you can buy Frozen Mallet instead of The Bloodthirster (Which will give you an extra HP boost if you're having Problems). I'd recommend you to have Clarity because Nasus often run dry of mana. And ghost for escaping or chasing =) please comment and vote, this is my first guide =)