First off, congrats!!!!! I hope you enjoy your job.
Second, I completely agree with what everyone has said here but a have a few things to add.
-If you ever find yourself stumped by a customer's question, don't hesitate to ask another employee that might know better than you, or in a worst case scenario look it up for them, most good customers won't mind the extra few minutes it takes and many will greatly appreciate it.
-Have alternative species. Say a customer wants a baby oscar for their 29g community tank. Instead of flat out telling them it won't work try steering them towards a similar species that will work, like a dwarf cichlid.
I hope these tips help and good luck with your new job.