you need a heater - eheim jager or hydor theo 50watts are good.
gravel or sand.
a lot of decorations like caves and plants - make sure none have rough edges on them you can check by running panty hose across them and if it snags so will your bettas tail
Basically, if you have the right lighting (flourescent 2watts/gallon) you can really use any type of truly aquatic plant and it'll be fine with the betta. Some good options are:: java moss, anarchis, marimo moss balls, java fern, anubias, rotala, moneywort, amazon frogbit, dwarf water lettuce, bacopa, hornwort, cabomba, crypts, amazon sword, etc. There really are a ton of plant options out there. I'd try to look for low light types though just because they are very easy to take care of.
Betta's dont need filters, but if you really want one look for either a sponge filter or something adjustable.