the "fish expert" was obviously not the "expert" he claimed to be.
welcome to AA! you have two options right now:
1. take the fish back and do a fishless cycle. this is the best option, and it will make your tank a much happier place for your fish when it is time for them to be added.
2. do a fish in cycle. this requires twice daily 50% water changes MINIMUM. only do this if you can't take the fish back.
now, if you pick 1 or 2, you'll need a test kit. the API master kit is the best one around, and using your own kit is really going to be the most convenient option. (you'll need to test daily for a few weeks on either option)
if you picked A, you need:
pure ammonia (no surfactants or scents)
fish food/raw shrimp
you need these in order to basically simulate a tank full of fish in order to sort of "trick" your bacteria into developing.
i also reccomend prime for A as well... just good stuff to have on hand.
now, if you pick B, you'll need:
a bigger tank... i would go 29 gallons MINIMUM, and that will only hold the angel for a little while...
prime water conditioner, because it detoxifies ammonia and nitrite for 24 hours while keeping it in a usable form for your bacteria.
preparation... this method is risky, and could possibly kill your fish.
i also reccomend live plants for this option... they help convert ammonia and nitrite, and are very good to have around during a fish in cycle...
i really hope we don't discourage you, fishkeeping is really fun after you get through this part, but hey... we all have to start somewhere