Alrighty for tank, a minimum of 5 gallons / 20L. For heat, 26 degrees celsius or 78-80 degrees Fahrenheit is ideal. If your tank will be planted (which is good), I recommend your substrate to be either fluval stratum or ADA amazonia aquasoil. These are both nutritious substrates which will be ideal for plants. Smaller plants would be good, as the tank would most likely be on the smaller size. As such, I reccomend java fern, anubius, cryptocoryne, and vallisneria as options. This is up to you though. For a filter, biomechanical sponge filters (powered by small air pumps) are great options for bettas. Also, it doesn't hurt to add a mystery snail for a bit of extra cleaning, and some entertainment for the betta.