In very simple terms (there are extra technicalities, which is why we have the beginner's resource forum; it would take too long to type every piece of detail every time!)
You need to 'cycle' your filter. 'Cycling' just means growing a colony of good bacteria on the sponge or whatever's inside your filter. The bacteria eat the ammonia produced by the fish and turn it into less toxic substances; first nitrite, which is also toxic, and then nitrate, which is only toxic at very high levels, and which we keep low with water changes, in a cycled tank.
You'll need; your test kits and a bottle of household cleaning ammonia (probably easiest to get it online).
Add enough ammonia to give you a reading of 2 or 3 ppm (parts per million) on your ammonia test (we have a calculator on the top tab, so you can work out how many mls you'll need).
Then you wait, testing every day, until the ammonia starts to go down. Once that happens, you top up the ammonia and start testing for nitrite as well.
When both ammonia and nitrite read zero on your tests, 12 hours after adding the ammonia, your filter will be cycled and you can think about adding fish.
D'oh; I forgot to say;
to the forum