Luke has given you the basics.
It is up to you what type of fish you want.
Cold water
Danios,
White Cloud Mountain Minnows (wcmm)
Goldfish, (includes, Comets, Fantails, orandas, Globe Eyes, Celestials etc...)
Paradise Fish
Corydoras...
... the list goes on.
Advantage of the above is that you don't need a heater in your tank or a light if you get a bit of light in the room already.
Tropical (warm water)
Angels,
Loaches,
Gouramis,
Killies,
discus,
the list goes on much longer for these.
however with these fish you do need a heater.
I personally would start with the following.
1 x Tank.... at least 15 litres.
Hang on Back filter...prices can start from $20
Gravel to cover the bottom.
Plants. (so the fish can hide in it.)
Go for Ambulia or cabomba
6 x White Cloud Mountain Minnows.
small container of flake food
With this setup you should see little babies in about a month.
and you don't have to worry about a heater or a light.
once you see the babies you will be hooked and wanting more and more tanks..... This syndrome is known by all of us here as MTS (Multiple Tank Syndrome) I am sorry to let you know that this syndrome is incurable.
You will also experience the law of No Money and small tanks... one day you will wake up and say to yourself 1200 x 450 x 450mm tank is just TOO SMALL.
At this stage your partner will leave you and your children will disown you.. the only friends you will have left are us here in the forums
Hope this helps.