The larger up to about 20 gallons the easier it is to manage. The reason for that is because fish have waste like you and I and with a small tank it makes it so you must do water changes more often because the water gets ammonia in it from fish waste and rotting food. The ammonia suffocates the fish. To prevent this from happening in a smaller tank you need to do lots' of water changes. A better and easier solution is to get either a 10 or 20 gallon aquarium. The larger aquariums allow more room for waste and allow what is called a biological filter to form. A biological filter is basically a bacteria colony that breaks down the waste into nitrite. When it is broken down into nitrite it is far less toxic to fish than ammonia.
The next stage of the biological filter is to break the nitrite down into nitrate. After you have had an aquarium for about 10 months this process will begin. Nitrate is the last form that the waste will take. Nitrate is even less harmful to fish than nitrite. After the nitrate gets past 1 PPM (Part Per Million) which can be read on a test kit you need to do a 10% water change. That is only when starting your biological filter. When it is established you have more freedom. You can do a small change of maybe 10% once a week or a 10% change once a month. The 10% change is much better for your fish than any larger amount.
As for buying your equipment select a retailer that you will stick with. It would be best to get a retailer that deals only in aquariums and fish for the widest selection of fish and equipment. If the tanks have dead fish in them or are diseased don't buy from that tank. Odds are that the store you buy from has an interlinked system that will spread the infected tank water around if one is infected but most good stores have a safeguard for that. Maybe it runs thru a filter before going to other tanks or it is treated with chemicals. Most good stores keep there tanks clean and disease free but sometimes disease is unavoidable. Most stores will refund your money if your fish dies as a result of a disease from there tanks. Most of the time it is a three day policy. Your retailer will tell you the equipment you need and what to do depending on the tank you get.