Hello Jenny..

I am starting into the aquarium hobby and was thinking of
getting a 20 gallon tank to start off with. However, I
have no idea what I need and what quantities to get.

A 20 gallon / 80 liter beginner's tank is fine!

I have seen what looks like a decent model: Aquaclear.
What do you think?

Aquaclear is o.K.!

What size would you recommend for my
tank?

Depending on these species you wish to keep. Some really
like water movement. The most lake species not really.

In what species are you interested in?

To keep the tank water at the right temperature will
require a heater.

For tropical fish, only.

The bottom of the tank usually has gravel for decoration.
How much is usually put in?

2-3" / 5-8 cm for living plants.

The tank I was looking to but is equipped with a single
15w flouorescent tube, is this sufficient light for my
fish.

Maybe enough for watching fish. Not enough to allow the most
plants growing well. Take a chance to start to the hobby
with an equipment that allows you keeping different plants
2x15W or 2x18w should be enough. Plants finally help to
keep water clean and most species feel better kept within
living plants.

I have also been told by the person at the store that I
have to set up my tank with a product called "Cycle" prior
to putting in my fish (1 week to be exact). Can someone
explain this to me?

Products like "Cycle" contain (not so many!, not
enough!) filter bacteria. But nitrificants finally exist
anywhere - for example in sand - and finally grow without
"Cycle". But that groth needs time. They grow slowly. So
please be patient and wait 3 weeks before buying the first
fish.

Anything else that I have overlooked?

Please study the internet, books, and finally choose fish
relating to _your_ water. Fish that peacefully coexist with
other fish.

Any other question? Than please ask.. ;-)
--
cu
Marco