I've been keeping various kinds of fish for about a decade. My suggestion is a single goldfish in an unheated and unlit 20 or 30 gallon tank with an external hang-on filter. I'd start on craigslist, and expect to pay about $100 for a newer tank in good condition with everything you need included.
1. Goldfish are pretty durable, cheap, attractive, and will thrive in room temperature water.
2. Larger tanks take a little longer to clean, but they less likely to have water chemistry problems that plague small tanks and kill fish. Do NOT get anything smaller than 10 gallons at the absolute minimum, and that won't be big enough for a single goldfish once it grows up. Those little 1 or 3 gallon kits are cheap, but very hard to maintain.
3. You could just swap out half the water every week and not use a filter, but putting a filter on the tank will greatly decrease the frequency at which you have to clean the tank. Hang-on filters are the easiest to clean and maintain. Do NOT get a filter that requires you to periodically replace their brand-specific filter inserts. They're like printer ink cartridges but worse.
4. You will still have to clean the tank sometimes. The more fish you have in a given space, the more often you need to clean it. So fewer fish in a bigger tank is easier.
5. Setting up a new tank means creating a new ecosystem. The tank needs the right balance of the right kinds of bacteria to turn the ammonia in the fish poop into nitrate. If you just put a PetCo goldfish in a tank full of tapwater, I'd expect it to die. If you know someone with an existing healthy tank, you can take some of their water and mulm (gross organic stuff from bottom of the tank) and put it in your tank, top off with treated tapwater, run the filter for a few days to clear the water up, and then try adding your goldfish. Still might die the first few times.
6. You can put special live aquarium plants in your tank to soak up the nitrate and then almost never have to clean your tank, but plants need special lights and they are expensive to operate and the lights will grow algae if you don't balance them just right, which means more cleaning. You also need timers and power strips and things to go with them, so I'd suggest just skipping the plants.
7. For about $15 you can buy a battery powered fish feeder that sits on top of your tank and feeds them twice per day. These work great, as long as you remember to keep them full. They're easy to forget about until they get empty or the battery dies and your fish start turning up dead of starvation. Basically, all of the food you put in the tank will eventually turn into filth you have to remove from the tank, so feed the fish enough to keep them happy without making them fat.