As advice for the future, you probably shouldn't buy champions who are less than 4800IP with RP. It just isn't very cost efficient. Don't waste a refund to get the RP back, but just remember that in the future.
My normal build is as follows:
Masteries: 9/21/0
-For offense, you want CDR, Expose Weakness, and the AD masteries.
-For defense, make sure you pick up Recovery, Perseverance, Block and Unyielding, Legendary Guardian, Oppression, Juggernaut, and Tenacious.
Runes: Quints - Health Regen, Marks - AD, Seals - Armor, Glyphs - MR
Items:
Against most matchups, you start with Doran's Shield, with the occasional Flask or Cloth armor purchase based on preference.
You typically want defenses against your lane opponent and CDR in your early items. Common purchases are Iceborn Gauntlet, Frozen Heart, Spirit Visage, or Randuin's Omen as your first item. After your first defensive item, you can typically buy Sheen, which I would recommend upgrading after your second defensive item.
For laning, I would recommend just staying near tower against lane bullies. You want them to push the wave towards you and you want to try and freeze it near the turret. You wither is an excellent deterrent against enemies diving you (wither whoever has tower aggro in most cases) and it an excellent CC if your jungler decides to gank your overextended enemy.
You have a lot of sustain between Perseverance, Health Regen Quints, Doran's Shield, and your innate lifesteal so you can endure most poke. Just be careful to let yourself heal up between exposing yourself to damage and don't allow them to all-in you.
Nasus has a low mana pool early, so I would recommend saving your W and E when possible. Though, if you happen to get an early level 2 or something, Q and E can allow for a strong trade.
Do not be afraid to miss CS early game. If you can't kill the minion without exposing yourself to a lot of damage, just make sure to be in experience range when it dies. Also, it is a bit situational but I have seen some Nasus players get an early Doran's Blade if they have the extra money after a resistance item (Negatron Cloak or Chain Vest) for some additional health, sustain, and (mostly) to help them last hit minions under the tower.
For damage, Nasus only really needs a Sheen item. Last Whisper is also good, but I would only recommend this after several tank items, if and only if you have Iceborn Gauntlet.
I believe that Mundo, Gnar, Olaf, Lucian, Mordekaiser, and Poppy can serve as good 'counters' to Nasus in different ways.
Mundo and Poppy scale just as well as Nasus and are better in mid-game fights, so they are fine trading farm and can fight Nasus are most points in the game. They also deal hybrid damage, so Nasus can't easily build against them, and can easily outtrade him at all levels (Poppy Q with Sheen, Mundo Cleaver with E-AA)
Gnar and Mordekaiser are both really strong at poking Nasus from range when he tries to farm in lane and are durable enough to fight him (Mega-Gnar, Mordekaiser's passive, E heal). Gnar and Mordekaiser are mana-less, so they can stay in lane for long periods of time, while Mordekaiser is also capable of giving your team a second ADC with which to kill Nasus during teamfights.
Lucian isn't a laning counter, but he is a lategame counter. This is primarily due to his passive letting him double proc Blade of the Ruined King and his E removing slows (including the attack speed reduction of wither).
Olaf is capable of easily harassing Nasus with his E, his ultimate will nullify wither, and he has sustain with his W. Also, Nasus doesn't really have burst damage for a good chunk of the game, so killing Olaf when he's low and sustaining through lifesteal can be somewhat difficult.
, I rarely ever build 
, as 
and 
can function better in most cases. In terms of boots, 
are an excellent buy, 
, 9/21/0, plus the new Elixir of Iron could stand out as a stronger buy once preseason starts, but its up to you. Damage beyond 
and 
, mostly for the armor pen, and CDR, 
as silly as it looks on 
, it can help you out-trade other top-laners when they have stacked hp, and 
, to aid in split-pushing.
, depending on your lane matchup, then build 
, 
, and 
. Ryze and Jayce have extremely strong harass, and will punish you for not being able to freeze a wave near your tower. If Darius manages to trade with you at all, you will have a hard time; ask your jungler for help in this matchup. Personally, i've found taht 
can wreck Nassus with his level 2 all-in, but he's not nearly as common as other top laners.
as a summoner spell so you can get back to lane quickly when you back early in the game, and so you can farm the top lane and then get quickly involved in a team fight.