I don't advise taking on a sorority for your first fish. People who have kept bettas for years have difficulty with sororities... they're a house of cards that can topple at any minute.
I think a 3-5 gallon tank kit (with light and filter) and ONE male or female betta would be a great choice. Make sure you do a lot of research on choosing a good betta, how to cycle your tank and keep your water quality good, what food to feed your betta, etc.
Here is an items list I think you need:
Tank with a lid and light
heater (a must)
filter (only if you are going to cycle)
a cave of some sort
aquarium salt (for occasional illness)
a soft sponge to scrub the side of the aquarium occasionally
bucket marked "fish only"
It would be really great if you got the tank and let your brother open that and then take him to the store to pick his own fish (with your guidance).. that way he gets the surprise of the tank and the enjoyment of picking out his own fish :)
*Be smart.. Do your research!!*