Here is my bit to add towards Mindemaes and Goldiegirls comments:
First I emphasize what Minde and Goldiegirl already said. DO NOT keep goldfish and other fish together.
To start with goldfish get yourself a comet or a common healthy ones. ( If you buy feeder fish, they most time already carry diseases... you can do this later when you are moe experienced )
Before getting any fish make your mind up how many you like to have and get the proper tank size ( 10 gal per goldfish )
You need a good filter, also an airstone and a pump, nice round pea sized gravel, live plants and some decoration. But please avoid everything with sharp edges or holes where the fish can get stuck.
Remember Goldfish grow quick
Then you need a very good waterconditioner, beneficial bacteria, pH chemicals. Pimafix and Melafix as treatments ( just like we have our painkillers in our home pharmacy ) good variety of foods and a gravel cleaner to syphon on a regular basis.
Now you can google for "nitrogen cycle in aquariums" "setting up a fish tank" also browse this forum, it was explained many times.
I say too, good luck, happy fish keeping and keep us updated