Welcome to AA!
How tall is the tank? If it is a 12" cube then it would be about 7 gals. That is a pretty small tank.
What do you plan to keep in it? That would have some bearing on the setup. You are fairly limited in suitable fish, so do some research to see what will work & what you like.
As a minimum, assuming you have just the bare tank, you would want:
1. Small heater - 25-50W. Get a submersible one as those are better quality. For a lot more $$, you can get a substrate heating cable & electronic controller so you can have your heating completely hidden under your substrate.
2. Small filter - a small HOB
is best. Something like an Aquaclear mini (or the 20). An internal sponge filter will work also (better for fish that don't like current) but will take up room inside the aquarium.
3. Light - A small clip-on light (fluorescent is best) so you can see your fish. If you go for live plants, you need much more than this.
4. Test kit - You need to read up on the nitrogen cycle (see article section) and get a test kit so you can monitor its progress. This is the most important step to keep your fish alive.
5. Dechlorinator - to treat water for use in tank. Most of us like Prime.
6. Supplies for maintenance & upkeep - bucket, gravel vac, net, algae scrubber sponge, towels .....
1. Substrate - ie gravel, sand etc. for the bottom of the tank. This is not optional for certain fish, and most of us find bare bottomed tanks uninteresting.
2. Decorations - some fish will need to have hiding places, caves for sleeping, etc.
3. Cover - You prob cannot buy a canopy or cover as this is a non-standard size tank. You can make your own glass cover easily (get glass shop to cut glass to size & purchase the plastic hinges, etc.) A cover reduces evaporation & keep in "jumpers".
this is a good source of info:
Beginner FAQ: Contents