Nasus is definitely tricky to play against if you don't entirely understand how his kit works but he's easily taken down if you know how to play against him. To start at the most basic level for full clarity, Nasus is a tank that deals AP damage and has a high amount of lifestyle built into his kit. He is very effective at winning via farming and excels at coming out on top of 2v1 situations, but with him having these advantages, he has things that counter him very hard. He's super vulnerable to constant harass and there are plenty of people that can beat him in a 1v1 situation.
As stated earlier, Nasus is susceptible to harass and is not the best duelist. So say that they first pick Nasus, this allows you to be able to counter-pick him to make the lane an easy win. For top lane, some good champions to pick against Nasus would be Gangplank, Teemo, and Pantheon. They all have ranged harass which keeps Nasus from going in on you and keeps him from being able to farm without giving you the ability to go in on him for a kill. The only thing Nasus can against this is two either use his E (Spirit Fire) to poke back which you can walk out of to avoid the damage or he can use his W to slow you down and fight you. Out of the 3 champions I listed, Gangplank and Teemo have abilities to reduce or ignore the slow Nasus has. For Pantheon, a smart Nasus is unlikely to go in on you since your passive can block his Q and you have much better fighting power at most levels during the laning phase, although his ult can turn a fight around in his favor. So if you're playing a champion with ranged harass, make sure to use it but not using it so much that you run out of mana. For a better duelist, punish him when he tries to farm and avoid the armor shred from his E.
If you're playing a champion that Nasus counters, it's the same thing as being counter picked by any other champion. Make sure you aren't overstepping so he can't slow you, avoid his E, and focus on getting as much farm as you can. If he's pushing the lane to your tower, ping it or tell your team he's overextended so your mid laner or jungler can come top and gank him, hopefully getting a kill out of it.
