first of all welcome glad you found us ,
Why don't the shops tell you about fish-less cycling?
they just sold you a tank filter gravel ect now what would make it complete.(Fish)
a lot of times the person helping is a new hire not knowing the first thing about keeping fish ,
you would expect they be trained with at least the understanding of the nitrogen cycle ,
this link should help you with the full details of the nitrogen cycle , if you have other questions feel free to visit the fresh water section as there are some pretty knowledgeable people there .
Guide to Starting a Freshwater Aquarium - Aquarium Advice
look forward to seeing you on the forum