The most important part of setting up an aquarium is that of purchasing the actual tank and all the necessary equipment to set it up. Here is where a lot of money is invested and advance research should be done to insure the money is well invested. For people on a limited budget, tank size is going to be the biggest factor on what can and can't be bought. For others who have a larger budget and a specific type of fish that they would like to keep, research on the specific needs of that type of fish needs to be done before any equipment is bought.
Getting the right size of tank and then making sure there is adequate heating and filtration is critical for maintaining a healthy aquarium environment. If the budget is limited and only smaller tanks are an option, be sure to stick to smaller species of fish that can live in smaller sized aquariums. Be sure to know the space requirements of the fish that will be living in the tank. Know the maximum capacity of the tank and always stay under it.