You need to fishless cyle the tank before getting fish.
You need to carefully acclimate the fish.
1 fancy goldfish requires 20 gallons, If you want more then one fish, your tank needs to be 10 gallons larger for each additional fish. And your filter should be good for almost double then the tank you are using.
1. Common, feeder, fair prize, goldfish need MUCH more room then fancy goldfish, a pond is recomended. These fish are beautiful, and hardy, but generally they arebred, handled and fed very poorly and you should not have high expectations for them. Ane please to not play the games at the fair that give them away as prizes, it is disgusting animal abuse!