Sorry we are confusing you Rat!
We know your tank is a new setup. Because it is only a few weeks old it hasn't cycled which means the filter hasn't established a bacteria colony. If you have a read at this it should explain things http://www.fishcrazy.co.uk/forums/t/19946.aspx 
You will read that fishless cycling is the best way to go and is kinder to the fish. A few flakes of fish food are added each day. As they rot and break down the nitrogen cycle begins. Filter start products can help, but alone will not do much. Some believe they do nothing at all. There are some other bacteria products like bactinettes which are more powerful though. Using frozen prawns is also good as they soon break down and produce ammonia.They smell quite nasty as they rot down!
Ammonia is the first thing to show then that converts to nitrIte. Both are very toxic to fish and can kill them. Finally nitrIte converts to nitrAte which although it can be toxic in large amounts, it is far less harmful to fish in levels below 50. It can even be ok in levels a bit higher than that. No water changes are done while the tank is cycling.
Once ammonia and nitrIte are nil and have been so for a couple of days, and nitrAte has risen higher than it was at the start, the cycle is complete and it is safe to add a few fish. The filter media has established a bacteria colony. It can take anywhere between 6 to 8 weeks for the average tank to cycle from scratch with new media etc. Some take longer some are ready sooner. A tank is still not fully mature though until it is at least 6 months old after the cycle was complete.
When a tank is cycled with fish a different approach must be taken. As fish waste breaks down ammonia and nitrIte levels rise and are harmful to the fish. So regular (daily) small water changes have to be done in order to keep the levels down. Otherwise the fish are in danger of dying. Some people still do cycle with a few 'hardy' fish, and might not lose any but some might lose them all. However,it is more common practice to fishless cycle an aquarium these days and no fish are lost.
In either case the water has to be tested in order to establish where the cyling process is. In the fish's case also to keep on top of those toxic levels. So, it's ideal to test the water at the outset so there is a baseline to work from.
All water companies add something or other to the water. So all vary as to what may be detected. All add chlorine which is very toxic to fish. And most add nitrAte so tap water might measure on the higher side to start with. So it's important to know what it is so you know when it has risen in the tank. You can still do that now and at any time in the future. Some tap water may also have traces of ammonia in it, but not often.
So the very basic test kit required is one to test ammonia, nitrIte and nitrAte. There are many on the market. dipsticks that measure some other things too all in one go. Tablets that dissolve. Liquid tests. I have used them all and prefer the liquid tests.
Yes each water change has to involve water that is free of chlorine and chloramine. Some people leave tap water in containers for a day or two so that the chlorine dissipates. But most of us add a water conditioning product to tap water such as Stress coat which also removes some other things such as some heavy metals.
Hope this helps
Please don't hesitate to aks more questions especially if still confused or unsure