https://www.kokosgoldfish.com/
This site gives a lot of helpful info on goldfish keeping, as well as a forum with helpful owners.
For twin tailed egg shaped (fancies) goldfish 20g per 1st fish and 10gs per each extra is a basic ratio used. I have 2 goldies in a 40g breeder.
For the single tailed longer (and get much larger) comets, commons, shibunkins etc common stocking is 40g for 1 and 20g per each extra-bare minimum 4' long tank but the longer the better for those.
I don't know how much you know about goldies or general fish keeping so sorry if I go over stuff you already know
POOP-WATER CHANGES
Be mindful goldfish make
a TON of waste-you need to do frequent water changes to remove the poop, especially if the tank do not have a pre-cycled filter, if it is cycled you may still go through a mini cycle. Most goldfish owners do a huge water change-draining 90%-just leaving enough to keep fish submerged and upright then refill (remember to dechlorinate and have new water same (or within 2 degrees F (1C) temp as tank water). You'll want to water change every 2-3 days to deal with the waste, if cycling wc every 1-2 days.
TEMPERATURE
Fancies do better in a slightly warmer water than the single tails you see in ponds-their digestive tracts are a bit ... odd from all the breeding that got them so egg shaped so the slightly warmer water helps them with digestion-keeping tank temp at 73-88F is good.
^Above info given from another goldfish owner, very knowledgeable with fish.
OXYGEN
They like their oxygen, I'd recommend getting an air pump and air stone/wand/stick for their tank.
FILTRATION
Lot of debate over this, look at the link above for info in, I use 10x turn over (400gph+ pump on a 40g breeder)
TANK PLACEMENT
These fish love attention, put the tank somewhere they can see you/others often (high traffic areas like family room or bedroom).
FOOD
They may look fat but don't have stomachs, feed small amounts of food. If you use pellets soak them in water before feeding (i just cup out some tank water and soak a pinch of pellets) give the pellets several minutes to absorb water and swell/soften, it also will make them sink which is good-floating food on the surface means the goldfish suck in air to take in food and they can get air trapped in them causing issues. I use Hikari sinking goldfish excel (the purple bag).
High quality algae wafers can also be offered (again soak first), and they really enjoy some of the gel foods like repashy soilent green-a lot of goldfish owners will get this food then add other foods to it like tiny bits of tuna, sea weed, various veggies).
They like veggies, offer blanched leafy greens like lettuce or spinach, or blanched cucumber once in a while if you'd like-they'll enjoy it. Fruits can be given as an infrequent treat-mine love oranges (just pee/cut off the membrane).
Don't feed them a lot of meaty foods, they're more likely to get constipated from them, brine shrimp are better than blood or tubifix worms. Frozen food is better than freezedried(more prone to cause bloat).
If you notice your goldfish have very long continuous turd strand reduce food amounts-they're getting to much. Poop should not be giant strands but a lot of little pieces.
DECOR
Be careful what you put in with them, they will try to explore every nook and carny and can get stuck in decor items-I lost one of my first goldies because it wedged itself behind a large river rock.. which then fall on it. Also if you keep them along enough to get to sexual maturity and finds you have a male and female, the male will chase the female around a LOT an d smash her into decor trying to breed which can lead to damaged/torn fins, missing scales.. infrequently a missing eye (more common in bulgy eyed breeds like telescopes/celestial) if there's something very pointy in there.
You do not have to keep taken barren but plenty of owners do.
SUBSTRATE
Many people keep a bare bottom (no sand/gravel) tank to make it easier to clean up poop, but if you want something at the bottom I'd suggest a small but heavy sand 9that sinks fast), goldies will sift the sand looking for food and if its too fine it can get inhaled into their systems and cause problems.. also they may eat gravel and have internal issue from this. I tried sand for a bit and went to bare bottom to make it easier.
LIVE PLANTS
yes you can keep live plants with goldies but they may try to nibble sometimes-tougher plants like anubias and swords are most commonly kept with them but I know a few owners that have some very delicate leafed plants in their goldie tanks.
The key to keeping the fish from eating plants is giving them another plant food source-feeding blanched veggies or the soilent green I mentioned earlier. They will always eat duckweed though, no matter how much soilent green they're fed, they like duckweed (duckweed is not nutritious enough to be only food source). Want aquatic plants but have a bare bottom tank? use terra cotta pots with gravel/small rocks for things like swords (add a root tab or soil for roots), or with rhizome plants like anubias and ferns you can tie them to decor, or suction cup them to the walls.
Remember faster growing plant=absrobs more nitrates.
Live plants are great for helping absorb nitrates from the goldfish, if you're worried about trying aquatic plants you can do a riparium using plastic shower caddies and most house plants (don't use bonsai/ever greens or cacti). Some house plants needs less light than others.. some need a lot-like aquatic plants. Pothos, peace lily, and wandering jew are some common low light house plants. You can see a large list of successful riparium plants here:
http://www.plantedtank.net/forums/25-riparium-terrarium-vivarium/951834-riparium-plant-list.html
Another option is aquaponics-goldfish are often used for aquaponics since they make a lot of waste to grow plants-its a great way to grow your own veggies in a healthy pesticide free environment. I'm doing this with my goldfish tank now growing lettuce and spinach, but you can grow just about anything.