Hi there emilythestrange,
I agree with Gilli that the more urgent thing is for you to begin performing rather large and frequent water changes.
The way you do this is to be sure you have a bottle of dechlor (this is a chemical that removes the chlorine or chloramines that the water company puts in to kill bacteria in tap water).. if you need to get this from a local fish store, be sure it just mainly does the dechlorination/dechloramination. API StressCoat and Seachem Prime are examples of products that could be used.
You siphon about half your tank water out. The return water should be treated with the correct amount of the dechlor product (per its instructions) and also you should roughly temperature match the return water using your hand to judge that it is close to the tank water temperature.
If you don't see your fish perk up and act a little happier, you can repeat this procedure as soon as an hour later, and so on.
Now, if you do have to go to the fish store then it will also be important to try and get a test kit as mentioned above. Take a look for the API Freshwater Master Test Kit or the Nutrafin Mini-Master -- the important thing is that these are liquid-reagent based test kits. Tests based on paper strips are not accurate enough for your problem.
Once you get a kit you can read the instructions and test your tap water and your tank water for ammonia, nitrite(NO2), pH and nitrate (NO3) and write up the results here in this thread for us to see. You should also record them in a notebook or on paper for yourself. You do not need to be buying other chemicals for your fish or tank at this time I don't believe.
~~waterdrop~~