So you will need a heater and filter and the rest is tank decor and completely up to you.
There is a lot of controversial betta info out there so getting it right the first time round can save a lot of heart ache.
Firstly no matter what anyone tells you bettas are tropical fish and do require a heater with a thermostat. Temperature fluctuation will cause a lot of stress to your betta and consequently lower his immune system and put him at risk of illness and disease, so a heater is very important.
Bettas also require filtration and good water quality. Because they have such huge fins they are prone to many bacterial diseases. This can be avoided by keeping their water clean by providing filtration and regular water changes. Partial water changes should be performed once a week, I usually change 30-50%. If you have gravel in your tank this should be vacuum siphoned every time you perform a water change as this removes the waste that causes most bacterial diseases if it is allowed to build up. As far as filters go I use cheap box/corner filters. They don't look the best but I find then suitable for bettas because bettas are not the best swimmers because their large fins weigh them down meaning that they can not tolerate a strong current caused by their filter.
As far as tank decor goes avoid anything with sharp or pointed edges as betta fins tear very easily and a subsequent bacterial infection may occur as a result of the injury. Plastic plants must also be avoided for this reason so always use silk plants or live plants as a safe alternative.
As far as tank mates go the following rules should be followed.
1. Keep only 1 single male per tank, males will fight to the death
2. Male and female bettas cannot be kept together the male will end up killing the female or sometimes as aggressive female may kill a male. They must be kept separate at all times except for short periods when breeding.
3. Several female bettas may be kept together if their tank is large enough however they are also quite aggressive in nature and groups of five or more must be kept together to avoid bullying and subsequent death. The occassional female may be so aggressive that they may need to be kept in their own seperate tank.
4. Keeping bettas with other species..... I prefer to keep my bettas solitary as I find they are happiest this way however if you do choose to keep your betta in a community tank then avoid any nippy species such as mollies, platies etc as they will harass your betta and nip his fins. Avoid colourful species like guppies that your betta may mistake them for a rival betta male. I also avoid catfish and snails as they produce large volumes of waste and therefore may cause your betta to become infected with a bacterial infection etc.
If you need to know anything else don't hesitate to ask