Funkytown is on the right track.
If you are a complete novice and wish to spend as little as possible then I would recommend that you start off with 'coldwater' goldfish'
and some live plants. looks better and don't have to worry much about them getting dirty
Step one
Cycle the Tank
This involves filling your fishtank with water and placing a couple hardy fish (comets, fantails, white cloud mountain minnows etc...cheaper the better) in it to get the good bacteria growing. this would take about a week. be patient and don't put in too many fish or they could all die.
Step two
Add fish
Go to your local fish shop:
Decor Pets - Civic Court, Napier Opposite the Library
Animalz - Taradale Rd, Onekawa
Taradale Pets - Taradale
With your size tank I would recommend that you have ony around 6 fishes.
Have a look a the different types of coldwater fishes to see what you like and purchase only a few at a time (ie 2-3 per week)
Take your purchase home and float the whole bag on top of the water for around 1/2 hour and then release into your fish tank
step 3
Maintenance
Most of us would suggest that you change around 20% of water on a weekly basis just to get things like amonia, nitrites and phosphates to an acceptable level.
another time to change water is when the fish look a bit sick.
Equipment required.
Filter
cheap ones are air driven filters
Medium priced internal filters
more expensive ones are hang on backs and canister filters.
Gravel
Enough to cover the bottom of the tank 1inch thick if you want to grow live plants.
Plants
Optional. anything you like the look of will do.
Lights
Optional. If you wan to see it in the evening then get one otherwise you dont need it.
Placement of tank Away from direct sunlight or you will have problems with algae growth.
No doubt others will also provide good advice or correct me where I have gone wrong. either way the most important thing is to have fun and enjoy the challenge of keeping fish.
If you have questions just ask them and we will help the best we can.
Cheers
Wok