The only things you NEED are a filter (so that the ammonia produced by the fish can complete the nitrogen cycle) and a heater (to keep the tank water around 75º... and depending upon your climate and setup, you might not require a heater).
From there, you need to decide how you are going to cycle the tank. Are you going to do a fishless cycle or a fish-in cycle?
A fishless cycle is where you basically setup the tank filter and let it run as you dose the tank with pure ammonia (a brand with no added soap or scents) to simulate the waste produced by the fish.
A fish-in cycle is where you setup the tank and add some fish, but you have to do daily water changes to avoid ammonia levels from reaching toxic levels until the tank cycles.
(In case you don't know, 'cycling a tank' is the process of allowing a colony of bacteria to grow in your filter to convert toxic ammonia produced by the fish into fertilizer. This bacteria lives in the air, so you don't have to go out and obtain any of this bacteria. But the process of growing a colony in your filter can take upto 2 months).
Other things to consider (mainly from aesthetics) is what sort of substrait (rock, sand, dirt) and do you want an air bubbler. The purpose of the bubbler, besides making mesmerizing bubbles, is to break the surface tension of the water to allow oxygen to dissolve in the water for the fish to breath. If you have a filter that is already disturbing the water surface, you don't have to have a bubbler... but I think the bubbles are interesting enough to always include a bubbler.
The other thing to consider (as you decide on a substrait) is do you want to do live plants?
Any specific questions so far?
