Welcome.
Here's what I would suggest. I probably left something out:
1. Filter. One that won't break. Make sure it is rated for 20 gallons.
2. Heater. It is safer to get two 50 watts rather than one 100 watt. In case one decides to go crazy. But of course one heater would be easier.
3. Water testing kit. Make sure it has pH, ammonia, nitrite, nitrate. API Freshwater Master Test Kit would be great.
4. Substrate. Rocks are good for a beginner.
5. Caves/plants for hiding. Reduce stress.
6. Fish net.
7. Gravel Vac. Keep your substrate clean.
8. Food. For your fish.
9. Algae scraper. Though this can wait.
10. Dechlorinator. So your tap water is safe to add to the tank.
You may want to invest in a quarentine tank. Do a search if you don't know what this is.
Now you will need to cycle your tank first before you add the fish you want. If you are unsure of what this is, I suggest you read up on the pinned topic here in the the "Beginners Questions." This is very important.
Good luck. Let us know how it goes.