Hi there GeorgieP and welcome to the beginner section
If your tap water is ammonia-free then you must be making your water changes too small! Since your ammonia level may indeed be 1.0ppm as you've indicated then the first order of business is to deal with this urgent situation. Start with a 70% water change, using your gravel-cleaner-siphon to clean gravel as you remove water. The return tap water needs to be conditioned (treated per instrucs with a product that removes chlorine/chloramine, like Seachem Prime or Amquel+ or API StressCoat+ or other such conditioner) and it needs rough temperature matching (your hand is good enough for this.) You can change water as soon as an hour later if this doesn't seem to remove nearly all the ammonia.
The next most urgent thing is for us to determine your test kit type. The paper strips are worse than useless, they are misleading. There are any number of good liquid test kits. Most of us, including myself, like and use the API Freshwater Master Test Kit, which includes tests for ammonia, nitrite(NO2), pH and nitrate(NO3). Once you get one of these or if you already have one, be sure to post up the stats for your tap and tank water.
Of course, the unfortunate, but common thing is that you were advised incorrectly about the setup of an aquarium. In the retail business, its typical for the shop to have a built-in fear of losing customers if they were to fully explain what's necessary to get things running fully and safely. An aquarium filter is really just a "kit" meant to be set up by an experienced aquarist who knows how to do the correct process to it for up to a month or two before its ready for fish. Its really pretty arcane what we do! We have to grow two separate but specific species of bacteria in the filter which help us carry out what's called the Nitrogen Cycle.
You can read all about it in our Beginners Resource Center. After you've done some heavy reading there be sure to come back with all your questions. We really enjoy helping other fishkeepers here, the members are really great!
~~waterdrop~~