Fish tanks require several different components, before you can actually add the fish to the water. Starting with a tank, you will need to look at hoods, lights, and filters. Talk with the salesperson about your options, or check them out online.
Gravel is perhaps the most beneficial substrate for fish, because it can help maintain the bacterial colonies that balance the pH level for the fish. Live plants can also keep nitrogen and carbon dioxide levels down while keeping oxygen levels up.
A tank heater needs to be purchased for the right size tank, because some of the smaller heaters can only manage small tanks and a large heater might overdo it in a small tank. A thermometer can be placed on the side of the tank or inside the tank on the floor, depending on the type you prefer.
A water vacuum will be necessary for those usually monthly cleanings. These do not need to be fancy or expensive and are very easy to use.
Also, for catching the fish and taking them out of the tank while you clean, you should consider a fish net and a small fish bowl with a narrow neck (because they tend to jump when stressed). Finally, use water conditioner to treat the water and ensure it is ready, with pH testing strips, before you risk putting any fish in.