10 gallons really isn't large enough for a gold fish. A 20 gallon will fit 1 or possibly 2 of the "Shibuken" breed of gold fish as they don't grow very large and swim slowly. Personally, I think those are better to have as pets as they tend to have really cool patterns, they're slower and have really elegant and long tails. They average around $5-$10 each but you shouldn't get more than 1 or 2 anyway. Don't mix them in with the typical gold fish which are faster and grow quickly.

Gold fish are part of the carp family and are social animals. They prefer to be around other gold fish so only have 1 is kind of cruel. Also, don't mix goldfish with other tropical fish. Goldfish get super aggressive around non-goldfish and also will literally eat anything that can fit it's mouth.

For substrate, don't get the sandy stuff as goldfish will accidently ingest it when they peck at the bottom. Get the gravel style substrate for the fish's safety and it's also easier to clean with a gravel vac.

Don't overfeed the goldfish which is a common and easy way to accidently kill your fish. They only need to be fed twice a day with a tiny pinch for each. Try not to crush the flakes and turn them into tiny little pieces, it will just get ignored by the fish and sit on the floor of the tank which pollutes it. Lastly, don't put real plants in the tank, the gold fish will eat it and die from constipation. Only put fake plants in there.

A much better alternative would be a betta fish. They are solitary fish, don't require much space (a 10 gallon would be on the higher end of tank sizes, while a 5 gallon would be ideal). They have really cool patterns and long flowing tails. A 5 gallon with real natural plants like Amazon Swords and Anubis would be good plants. You only need to change like 1 gallon a week so maintenance is very easy and only takes a minute. Bettas come from warm enviornments though and need a consistent 70 water temp.