I would get a 40 gal top filter and an undergravel filter. The combination of the two made one of the cleanest tanks I've ever had. But as some people said, no matter how good your filter is you need to clean it. I could go 2-3 months before it would need a cleaning.

I would get a heater, most fish are tropical and need some heat. They are pretty inexpensive and all you have to do is make sure the water stays high enough.

Well you have the option of getting a wide variety of fish. One thing many beginners do is buy a whole bunch of expensive fish for their new tank and then a few days later they are all dead. I hate to say it but usually first fish that go into a new tank are "sacrifical". Fish produce bacteria, some bacteria they need to live, eat, breath, and poop properly. This bateria is usually spread through fish poop. So until it levels off some fish are going to die.

An easy way to avoid this is buying a prodcut called "Cycle". It pretty much puts the bacteria in the water for you.

Some easy fish:
Clown loaches- Easy but a tad expensive, They need schools of 2-3.
Angelfish- Pretty but semi aggressive
Tiger Barb- Semi aggressive, but most barbs are.
Plattys- Easy and come in a billion colors, but breed so easy. Left unchecked they can take over tanks. Same with guppies.
Gourami- Pretty fish little aggressive

There are many many more. Also have your watch checked before you buy fish. I think any place that sells fish does a water check for free.

Sorry for the length, but I hope it helps.