The first thing about this hobby is to never take the word of a single source - such as the "lady at the per store". She should be considered as having one strike against her already for leading you in that direction to cyle with fish - especially after hearing you were considering the raw shrimp route. Shame on her. Get a good test kit and watch that ammonia. May need to do more frequent water changes as not to kill the fish while cycling.
Better still, consider taking the fish back and doing the shrimp thing.
Most importantly, read some start-up information. Robert Fenner's"Conscientious Marine Aquarist" is a good start. Ignore the part in it about cycling with fish though.
Good luck. But be careful of "that lady".