Keep in mind that you need to do 100% water changes in your 5 gallon once a week unless you choose to filter and cycle it. The "training" people get at pet stores is abhorrent, in my opinion, and it's a terrible shame that that misinformation is passed on to the customer as well. The simple truth is that the pet store makes more profit off of a low cost made-in-china dinky plastic betta coffin than they do proper tanks with a higher cost. The sooner you realize that your value to the company isn't your accurate knowledge of proper pet care, but your ability to make the company money the better. It's not just fish either, any small animal, be it a bird or a hamster or a ferret or a hermit crab--you can expect all of the care information they gave you to be terribly wrong. If as many people understood the proper care requirements of these animals as wanted them, they would not be nearly as common as they are now.
I suggest researching the nitrogen cycle since cycling the tank will make your life a lot easier in the long run. If you work at a pet store with fish tanks, you should be able to get some filter media to help seed your tank easily, which will speed the process along quite a bit.