Hello and welcome newbie emma.b.
The most important thing to do overall is to start reading about the care of your fish.
There is a link in my signature which will really help you.
Do you have a test kit, or can you take the water to the pet store and have them test it? A new aquarium is going to be fluctuating with the amounts of Ammonia that is created by the fish waste and food turning to ammonia and the BB if you had some would help convert that to NitrIte and then NitrAte. Called the nitrogen cycle. Also referred to as "cycle" the tank.
You have 2 basic options, return the fish and try a different option as in a "fishless" (as in no fish in the tank while you build your BB) cycle. Or keep them and get a test kit and do a hundred tests and water changes called a "fish in" (as in, fish in the tank) cycle.
Here is a helpful link for you.
Fish-in Cycling: Step over into the dark side - Aquarium Advice
That all said, 2 Platies are not the best selection for a 19L, though 2 Goldfish are a horrific option.
You might think about a Betta for the 19L. Or if you like the Platy then maybe a larger tank a 20G long /75-76L would be better so they would have plenty of room to swim and you could get a few other fish.
As for the immediate I would do a 80% -90% water change and match the temp as best you can and always use dechlorinator and don't feed them today. Flake foods turn to ammonia very quickly.
If you can get Prime water conditioner that would be the best. It helps neutralize the water of ammonia, nitrate and nitrate poisoning.
Edit - I see you have already been offered good advice! I am too slow!