What you should take purely depends on preference, playstyle, and your matchup. Though, there is one build that works. Taking the standard Fleet runepage with the normal crit build usually works in most cases. If you want a bit more survivability, then take the bruiser/crit build for a good mix of health and damage. You could also just build what normal top lane Nasus builds in most cases.

You are probably the weakest champion in your team early game. Focus on stacking and farming to scale your strength. Because Nasus is so weak, you always want to make sure you get a leash. Be sure to put a second point in Q at level 3 to help with your clearing.

Once you've done a full clear, you've got two options: Try and go for a gank, or continue to farm. With his wither, Nasus is actually pretty decent at ganking lanes. You probably won't get a kill this early on, but you can definitely make enemy laners at least burn their flash. The best option though would be to just back for a sheen and continue to farm. Without sheen and boots, Nasus isn't very good at securing kills. After you buy a sheen, focus on your tier 2 boots. Once you have both of these and Ult, then you can go for a gank or two before you must go back to farming.

With a few hundred stacks, an item and your Ultimate, you should always be trying to take objectives. Nasus Ult makes taking dragon a breeze. Hell, you could even take dragon as soon as you reach level 6. You can take fights a lot more now, and 1v1's are practically free stacks with your Ult up, but you should still be prioritising stacking for even more damage.

If you do succeed in pulling off a gank, then you could most likely just take that lanes tower right there and then to gain prior and let your laners roam. This will make getting objectives a bit easier.

In the late game, you are a damage god. Any squishy who gets within 10 feet of you is gonna die. However, the enemy team is most likely gonna prioritize killing you above anything else, so try not to pick fights solo, cause if the enemies are smart, then they're gonna try to all stick together. Stay with your team during fights to try take heat off yourself.

As for towers, you destroy them with ease. You see a tower with no enemies? Take it. It would only take you like 6 seconds.