On the ammonia: this must be in your tap water. Have you tested just the tap water on its own? If ammonia is present, I would recommend a water conditioner that detoxifies ammonia as well as chlorine and chloramine. Not all do, check the label.
On the quarantine: once you have an established aquarium, quarantining new fish is wise. But in the initial setup with the first fish, I myself wouldn't bother, since this is a smallish tank and there is nothing in it yet fish-wise.
As mentioned by others, float the bag for several minutes (15-30) to even the temperature; sudden temp changes can severely shock and stress fish, so this slowly gets the bag water to the temp of the tank. Then open the bag, being careful not to let water out into the tank; if the bag is quite full, I remove some of the water and discard it. Then use a small cup or something similar to add some water from the aquarium into the bag, maybe a cupful. Wait 10-15 minutes. With hardy fish this is usually sufficient, but with sensitive species I repeat this step, sometimes twice more. When ready, use a small fishnet to transfer the fish.
You will need more plants first, to handle the initial ammonia from the fish. If not, another way to "seed" bacteria is with a good bacterial supplement. There are two I know of that work. Seachem's Stability and Tetra's SafeStart. A small bottle of either will be sufficient. These products contain live bacteria to jumpstart the colony.