If your tank doesn't come with a heater unit, then the only fish you can put in there will be coldwater fish, not tropical. The commonest coldwater fish and a good starter are goldfish. You can get fancy goldfish types as well, 2 of mine are fantails [self-explanatory when you see them] and one is a rhyunkin[sp?]. These are usually red, but can come in any colour and look as if they're pregnant [egg-shaped belly], but they are quite hardy.
Keep adding the stresszyme as instructed and you need to have the tank going for at least 10 days IMO, better is 14 days to be on the safe side, before you even introduce any fish. I'm assuming that you have the essentials such as the air pump and airstone, water filter etc and that you've treated the water with something like Tapsafe or Aquasafe, to remove the chlorine. One other tip I would suggest buying some oxgenating tablets. If the power supply fails or your air pump breaks down, you put a few of these in the water and it will keep them going for a while.
Go to a reputable dealer who will guarantee the fish and give you a refund or replacement if they die. Pets At Home offer this, as do Maidenhead Aquatics [only in the London area]. Our tank is about 30L and we have 3 fancy goldfish in it, all of whom are about 3" long. Coldwater fish will grow according to the space available to them and however much they are fed. I would start with 2 fish and see how you get on. When you bring the fish home, remove some of the water leaving about 2-3" from the top of the tank. Place the unopened bag into the tank and let the water inside the bag acclimatise for about an hour. Open the bag and dip the neck into the tank water, so that the fish can swim into the tank. Don't feed them until the next day.
If the tank has an interior light, switch the lighting controller on and plug into a timer. Set it to come on at lighting up time and go off when you go to bed. If siting the tank near a window, make sure to pull a curtain across if sunlight is strong, as this leads to green algae on the glass. There are all sorts of other water treatments on the market. Tetra Easybalance is just one, which is supposed to reduce the need for water changes. Be guided by your manufacturer's instructions and your local aquarium suppliers who will be able to give much needed advice.
Good luck with your endeavour, hope everything works out ok for you.