So Nasus is a fantastic champ, and this is just a little advice on how to play him, and my build for him. It is very similar to DavidSS's
build (His build is what got me started, and since then I've modified it to my style).

So Nasus is a great champ, and you're going to rack up lots of kills and assist with him. However, he's not a carry. You have a very aggressive support role. With Wither and Spirit Fire, your job is to slow them down, and to whittle them down so your team can finish them off. Wither is a great slow, and Spirit Fire, does good damage if they're dumb enough to stand in it, or are stunned or otherwise unmoving.

In the early game, and onward, your main focus will be weakening creeps with Spirit fire, then coming and cleaning up with siphon. This is very important as Siphon gains permanent damage increase every time you kill with it. So focus on getting good at last hitting with it, it also allows you do double hit with it. It will strike almost immediately after a basic strike so work on timing.

Get Siphon first off so you can start getting that damage with it. Then Spirit Fire will become your best farming and crowd control technique. Then get wither, so you can start taking down those champs and preventing from running away.

I'm still working on which runes I want with him, so I'll update ASAP.