Your first strategy of every game as Nasus is to survive. Stacks are secondary. Nasus will always scale but if you feed a snowball champ a kill you may have not have only doomed your lane but your team's as well.
ALWAYS max Q first. Exceptions are against extremely hard ranged match ups like Quinn, Vayne and Teemo. However all of them are extremely easy to gank with a jungler and wither. But if you get no help you may want to max E first to at least get some gold and EXP.
TF is strong but not optimal for Nasus. Typically you will be harassed a lot with little chance to farm at your leisure meaning Gold is limited. IBG provides more optimal and gold efficient stats then TF. It gives Armor, Sheen proc and that beautiful 20% CDR. The slow Zone adds some extra utility and damage as well. With IBG and SV as your rush you can essentially beat anyone in the game if you did not feed. A lot of people will argue with me on this but Thornamail is a great 3rd or 4th buy. If you get this before the enemy has pen and life steal they will kill themselves on you while you heal for free.
From there build mercs if you need tenacity or tabi if they are mostly AD/AA reliant then build health/resistance items. Titanic Hydra and Steraks are good buys on Nasus almost every game as well. Do not forgo them unless you absolutely need the resistances from other items more.