Its a common myth that they dont need heating. Ever wonder why fighters in petshops always look dead, sitting on the bottom of the tank? Thats why. They are actually quite an active fish when properly heated and its wonderful to watch. Best temps are around 25-27 degrees.
They also like still water, this means you shouldnt use a fast flowing filter. I dont use a filter at all in my setups, I just use large rocks that I can easily take out and wash (instead of the tiny gravel) , and clean the water once a week. I use rainwater, it sits at the PH they prefer, and then I dont need to worry about dechlorinating.
I also recommend the proper betta food pellets you can buy, it doesnt seem much in the packet they come in but considering they only eat a few a day, it lasts a very long time.
As for tank decorations, anything you prefer really, but give them something they can hide behind if theyre feeling shy