The betta should be pleasing to the eye. The finnage should be even. It should be active and curious. Bypass the lethargic ones - when you pick up the cup the should become aware of you. I suggest getting a
Marina Betta Kit when you purchase the betta. These kits come in red, purple, and lime green. You can get one to contrast the color of your betta. There is a light you can purchase separately for the kit. The kit comes with gravel and a plastic plant. Wash the plant, stick it to the floor of the tank, rinse the gravel thoroughly, place it in the tank, add water and conditioner and let sit for about 1 hour. The kit also comes with food pellets, but your betta might reject them. Then add the betta.
You may also need Stress Coat or Splendid Betta Complete Water Conditioner when the BettaPlus water conditioner that comes with the kit runs out after doing the weekly water changes. (The BettaPlus water conditioner seems to last for about a month's worth - it suggests a 25% water change once a week)
Maybe a small container of freeze-dried bloodworms and a face shield for when it spits out the food pellets at you.
Good luck!