It really is awful what people will put fish in. Betta and goldfish are the most abused fish. Goldfish because people don't realise how big they get and Betta because the can survive in little water (survive, but not thrive!)
On the topic of tanks, avoid ones that are taller than than wide. Tall tanks have less swimming space and less surface agitation.
I used to feed mine Betta pro. Great food, but expensesive (I used to breed them so needed high quality). Anything but flake is fine, and if you can get one especially for Betta it's even better.
I'd avoid colourful gravel. It's eye popping but your fish won't really appreciate it and it'll take the focus off your beautiful Betta. Fine gravel or sand is great for plants
With a Betta you'll want to avoid anything with rough edges as they can easily rip their tails on them. I know in pets at home you can get half coconuts with anubias or javafern readily attached. Those are pretty cool. Or you can go heavily planted and let the plants make a natural hide. Or have a look what hides you lecal store has and pick a theme. Their are lots of choices

Yes, a light source will be needed, especially with plants! Most tanks will come with one but they are easy enough to get for a small tank. And the beauty of getting "easy" plants is that you don't have to worry about bulbs and the types of lights they emit!
Betta are fairly easy to sex. For the most part at least.
Males have the long flowing fins and bright colours. They will also be housed on their own.
Fun fact: in the UK it is illegal to keep two male Betta in the same tank. Females in pet stores tend to be quite dull in comparison. Usually brown with a blue/red iridescence. But you can sometimes find colourful ones. However, they will always have short tails.
Wild colour female example:

Of course, there are a variety of Betta called Plakat (my personal favourites) where both male and female have short fins. They're a lot harder to come by though and if you find a colourful short tails Betta in a tank on its own its most likely male!
Plakat male:
