Bah! Ok.. I got given a tank.. about yay big.. .. Anyway.. It has two gold fish in it, three plants, some gravel and some kind of air putting in thingo. (As you can see I have no idea about all this.. )

What do I need to do to ensure they don't die?
Do I need a filter? The people who gave them to me said something about water changes..
It's not a very big tank.. maybe 60 cm x 30 cm x 20 cm max.

Thanks everyone in advance for any help you might be able to give me...

God no I'm stupid.. tank is 30 cm long, 18 cm tall and 17 cm wide.. I think that makes a difference.. (As you can now tell.. My powers of estimation suck)

If the measurements you have given are correct then your tank has under 3 gallons water capacity. It is generally recommended that each individual goldfish has at least 10 gallons in order to grow properly and stay healthy so your tank is very overstocked. It would be great if you could get them a much bigger tank but until then you will need to do 50% water changes daily (using dechlorinated and temperature matched water) in order to keep the water quality fairly good.
A filter is essential due to the amount of waste goldfish create, but in that size tank you will be fairly limited as to what you could have. A Fluval 1 internal filter would be ok, as would a sponge filter with an air pump.
Dechlorinator is a must (one that removes chloromine too if you have this in your tap water). Test kits for ammonia, nitrite, nitrate and PH are also crucial to keeping good water quality. You also need a gravel vacuum to make sure your gravel stays scrupulously clean.
Good quality sinking pellets to feed them are generally better than flakes, as well as bloodworms and brineshrimp (dried or frozen) a couple of times a week. Supplement thir diet with just about any fruit or vegetable (chopped and cooked if hard) to give them variety and extra vitamins.

That tank is actually a 5.5 U.S. gallon tank. I have on that i'm going to be using for a betta, tried keeping mollies and other livebearers but its just too small. An AquaClear Mini filter is quite inexpensive and easy to use, would fit that tank, but the fish will outgrow it quickly.

Wow.. ###### hey.. That's bizzarre.. there was a stall at the big Westfield centre that had baby Siemese Fighting Fish in apparently self-cleaning tanks about 15 x 15 x 15 cm..

the siamese fighint fish are betta, and they don't need much oxygen in the watter if at all,because they get oxygen from the air if i am not mystaken. but Gold fish in general are a differnent spiecies all together! 1 the poop alot 2 grow quickly 3 are way better then bettas(o wait did i say that) . so in that tank you will need alot of filtration to keep up with the gold fish...but if they are still lik yong 1-2 inches then they can live there for a while but eventually you wi8ll want to get a bigger tank. The only problem you have is that you will want to be concerned with your watter quality... so get a test kit and then do the steps occordingly

Cheers

There's no such thing as a self cleaning tank.

Siamese fighting fish do great in "smaller" tanks, since they max out at ~3" and don't produce a lot of waste (compared to goldies). Goldfish can get to be 7" or bigger.

Definitely do get a filter. If you can get a bigger tank for these guys, that'd be great, too. Keep the layer of gravel low (~1/4"). Read up on what it means to cycle an aquarium and pick up some test kits (ammonia, nitrite, nitrate, pH). Water quality is THE most important thing for goldfish. If you learn how to take care of your water, the fish will take care of themselves.

Some1 posted somthing on a self cleaning tank. It looked like a bowl.. and it would "somehow" clean the tank by itself. So all u had to do, is feed it, and it would "supposabely" stay alive for a lng time, which is BS

I think a self-sustained eco-sytsem is possible in a tank but i also think it requires a much bigger tank with LOTS of plants and a very small amount of fish. That fighting fish setup you saw wouldnt be self sustainable for very long.