There are different ways to do this; one is to buy the tank and find what fish can live in it or the other is to look at fish you like and find what size tank is needed for them. Either way is fine but you started on the right track by doing your research first.
I would also start by considering where you are going to place your tank, consider the maximum size that will fit the area, the weight requirements-if this is going upstairs for example, will it get sunlight? this can make the hobby difficult dealing with algae issues.
Also think about what will your budget allow? How much will it initially cost to set up and also to maintain, including electrical and water.