Cycling is one thing you do need to get your head around because if you just add fish and do nothing, the fish could well die.
In simple terms, fish make ammonia as their waste. It goes straight into the water and unless there are any bacteria to remove it, it builds up and burns the fish's skin and gills, making it hard for them to breathe. Eventually, some bacteria grow in the filter which use ammonia as food, and they turn the ammonia into nitrite. As more and more ammonia eating bacteria grow, the nitrite level builds up. This is also dangerous for fish as it binds to their blood stopping them absorbing oxygen. Eventually another type of bateria grows which eats nitrite and turns it into nitrate. This is only toxic at high levles, and once the filter has cycled (grown the two bacteria) we remove it by doing weekly water changes. The trouble is that these two bacteria grow very slowly, several weeks is the usual time to grow enough.
The safest way to grow the bacteria is to add ammonia solution with no fish in the tank so the bacteria have grown when you finally get fish. But it does mean you have a couple of months with an empty tank and I can hear your parents asking why there are no fish in it yet

The alternative is to cycle with fish in the tank. This can be done if you are prepared to do the necessary water changes. With 23 litres, it won't be hard. I have a 25 litre tank with a betta, and when my previous one got finrot after his tail was bitten I did daily water changes for three weeks to keep the water extra clean while he healed. So I know it's not too hard

Whichever you decide to do, you'll need a tester for ammonia and nitrite. You can get strips which you dip in the water and look at a colour change, though you need to make sure ammonia is included as a lot don't; or there are liquid reagent testers where you put some water in a test tube, add drops from reagent bottles, shake, wait a few minutes and compare the colour to a chart. They sound complicated at first but they are quite easy. I have the API liquid kit. Tip - on-line is cheaper!
With cycling with fish, you test the water every day and do a water change if there is a colour above zero for either or both ammonia and nitrite. The aim is to stop them getting above 0.25.
That's another thing you'll need - a siphon tube. I would advise you to get one before you have fish as they can be a bit fiddly when you first start so you can practice without any fish in there. Some you shake up and down to start the siphon, others you squeeze a bulb to start it, and then there are somewhat more expensive battery powered ones. Have a look at what your shop stocks. You'll need a cleaner like this during cycling and to do your weekly water changes after the filter has cycled. If you have gravel to go on the bottom, you push the wide end of the tube into the gravel, watch all the fish poo being sucked up then move to the next bit of gravel. I promise it gets easier with practice. The first few times you'll only clean a bit of gravel before you nearly empty the tank

And you'll need a bucket or two, one for the dirty water to go into, and maybe a second for putting new clean water back in the tank.
Did you get a bottle of dechlorinator with the tank? If there's a bottle of Nutrafin Aquaplus, that's it. You'll need to add some of that to new water every time you add it. It'll only be a small bottle, don't foget to buy more before it runs out.
There might also be a bottle of Nutrafin Cycle. That is supposed to add the filter bacteria. The problem is that a lot of people find it doesn't work. If you've got some, by all means use it but assume it won't do anything. That way, if it does you'll be pleasantly surprised

Sorry, I've rambled on a lot there. I started out by trying to say, don't worry. Yes is is a steep learning curve but once you get into it, you'll want another, bigger tank. I currently have three and the only reason I don't have more is that my husband doesn't like fish!