Betta Fish are tropical fish, which means they like their water warm and from 76-80 degrees. Get an adjustable heater like Hydor THEO or Ego Jager.
It's a common misconception that bettas like small spaces, this is not true. They like the space, and they need exercise just like any animal. I don't recommend any tank size below 1 gallon and remember to do your water changes! Ammonia would build up very quickly in tanks so small so get the biggest tank you can get. Remember to get a good water conditioner that extinguishes heavy metals, chlorine, and chloramines.
As for any animal, variety is key in any diet. However, every diet needs a staple food. For bettas, it's usually high protein pellets. Get a good brand such as New Life Spectrum, Omega One, or even Golden Pearl. NLS and Omega are the ones found in stores. Got your staple? Good, on to your varied foods. Freeze dried, (FD) frozen, or even live foods are good treats to feed. Remember to soak your FD foods, and wash your frozen and live foods. You don't want parasites to overcome your fish! And last but not least, fasting and peas. Peas are more for goldfish and bettas are carnivores. They don't really need veggies in their diet. You have an option to fast 1 day per week so you may do so. Fasting technically means not feeding..