What do I need and how do I take care of my new fish?

I just bought 4 fishes, 1 angel fish and 3 neon tetra, with a PETCO mini aquarium kit that is only a gallon, I put some of the gravel in and I have been filtering the water for almost a day. The fish are still in the bag floating in the tank but I don't know when to put them in or even how to take care of them. Do I need a bigger tank? I have been researching the internet and I have noticed something about testing the water and getting a bigger tank but the lady at PETCO said that the 1 gallon should be fine. I don't know what other supplies I need, like do I need a plant too? And how often do I partially change the water? How much water?