if your frets are even and your action isnt too low, and your nut isnt the probalem, then its the truss rod.

you need to leave the neck with a little bow but it shouldnt be big... i mean a very small bow so that there wont be fret buzz. if you make it perfect straight, buzz will always be there. leave it with a little back curve...

and if your music shop told you to fix it yourself, then maybe they dont know what theyre doing.