I might be able to assist you there is SO much to tell you so I'll just answer your questions for now and give you a list of things to do at the end.
Right, first things first I have some bad news- your tank isn't big enough to keep fish in. I know that all pet shops sell those little beginner tanks and it's a real bugbear of mine because small tanks are really hard to keep fish healthy in. You shouldn't really consider anything less than 45 litres, but closer to 60 litres is an ideal size for a beginner (this would be approx 24x12x12 inches). The tank you have is only 20 litres.
Tap water is OK for keeping some fish in but you have to add conditioner that will remove the chlorine. You will also need to test a few things to make sure you get the right fish to match the water.
And you can get timed feeders for fish too they are much better than those slow release tablets that just mess up the water.
here's a list of things you need to keep fish successfully-
A tank of around 60 litres, doesn't need to be new or expensive as long as it doesn't leak!
A filter (fluval 2+ is a good size for a 60 litre tank)
A heater if you want to keep tropical fish (50 watts for a 60 litre tank) and a thermometer
A water test kit (this is absolutely essential!!)
FoodWater conditioner (like tap safe or aqua safe)
Cleaning equipment- syphon tube, bucket and gravel vac
Plus all the gubbins like gravel and plants and a 'no fishing' sign
It is quite complicated to start with but once you've got the basics then you'll be fine. The toughest part is choosing the right equipment and fish especially since shop staff are rubbish at giving advice 99% of the time. It's best to do your own research, if you just google 'fishkeeping sites' there are hundreds.
It can be an expensive hobby too but you can always get loads of bargains off ebay and freegle.
Hope that helps a bit, let me know if you need to know anything else