Um...
this seems like a good place to start.
Tank Size
I would agree that 2.5 gallon is a good starter. If you are short of space and can only fit a one gallon somewhere, be prepared to take the time to dechlorinate tap water and do water changes every week.
Betta Type
If this is your first time keeping a betta, its advised to start with a veil-tail or plakat if possible, those tend to be hardier and better for beginners.
Needed Basic Equipment
Tank
Filter + air pump
(to be honest, I have kept bettas without a filter or air pump, but its good to get if you can find pre-packaged tank + filter kits)
Heater (if you're not in the tropics)
Water conditioner / dechlorinator
Betta food
Dried blood worms
Advised accessories
Gravel (not more than 0.5 cm thick if you're using fine sand. Anything thicker risks anaerobic activity)
Plastic plants without sharp edges
Nitrogen test kit
Nitrite test kit
Keep in mind that if you do decide to get a fish, you'll need to buy the tank and equipment first and 'cycle' the tank. Cycling takes 4-6 weeks thereabouts, so you'll have to get your fish 1 to 2 months later. If you want to buy all the stuff at one go, be prepared to keep your fish aside in a smaller bowl and do near-daily water changes until the tank is properly cycled.
Cycling a new tank
I normally buy all the stuff at one go (or rather, already have it lying around).
While waiting for the tank to cycle, I do regular (2-3 days) water changes to the rescued fish. However, instead of dumping the fish's waste water away down the sink, I put it into the cycled tank! This gives my cycling tank the regular waste nutrients produced by my fish (for bacteria to om nom nom) without the risk of damage to the fish's delicate gills when the peak in toxic nitrogen/ nitrite occurs.
4-6 weeks later, when the cycling is complete, I introduce my fish to the tank.