First off, Petsmart employees know nothing about aquariums.
Now to your question. An established tank is a tank that will turn ammonia into nitrites and nitrites into nitrates. Bacteria living in your filter does this. Ammonia and nitrites are very poisonous to fish. Nitrates are less toxic. It can take anywhere from 2 weeks to cycle, to my tank taking 5 months.
While your tank is establishing you need to constantly check water parameters with test kits (I highly reccomend API liquid test kit. 18$ on amazon, and will last you years). If ur ammonia or nitrites go above .25, do a waterchange. When ur nitrites and ammonia are 0, with nitrates present, ur cycled. Anymore questions feel free to ask.
Oh wow. Yeah they didn't tell me any of this stuff!! Thank you so much!!!