I think this is a good time for you to start planning your tank. First of all, what fish do you want to keep? It is true that if you keep certain fish like corys you need to think about the gravel. Ideally, they want sand; second best is really smooth rounded gravel.
Are you going to have live plants? They can be planted before you bring the fish in.
If you don't know how to set the tank up, can you get help? Even go back to the shop and ask them for help.
At the same time look at their fish and see what you want. Make a list of what looks attractive and then go home and do an online search of the species to see if they would fit the tank you have, how big they grow, if they have special requirements, if they are territorial or can be kept with other fish. Then, before you buy, come back to the forum for a last check- there will almost certainly be people here who are experts in keeping the fish you fancy. A 70 ltr tank is not enormous so you will be looking at small to medium-sized fish who are not very fast swimmers.
A fish tank will not be ready just because you have the filter and lights working. It also needs to be cycled, i.e. for friendly bacteria to grow who can cope with your fish's waste. The two main ways of arranging this is to start with a few hardy fish and add more gradually after testing the water, or to do a so-called fishless cycle, where you add household ammonia (from hardware store) to the tank for a few weeks before you add the fish. Again, you need a test kit to see when the tank is ready to take fish.
Most of us would recommend the fishless cycle as it is kinder to the fish. Read through the links carefully and then come back with questions.
If you to decide to go for cycling with fish you need to be very careful in choosing hardy fish and checking the water stats so that they do not get too stressed-out.