*eyes username* Your name makes me think I've seen you somewhere else around the net.... *ponders*
Does the tank have a filter? It's not really needed (especially if you aren't going to be cycling it) but it's a handy thing to know. A heater is indeed more important to a betta. They are tropical fish and need warm water, at least 78 degrees. Pick one up the next time you can swing out to a pet store (or Wal-mart, they carry heaters too) and you'll notice a betta will be far more active in warmer water.
Bettas and mirrors. Bettas are extremely aggressive, territorial fish by nature, and they will flare, chase and try to attack any other betta (male or female) they percieve as a threat. A mirror is a fun thing to put up against the side of the tank and let them flare at several times a week. Many bettas will even play chase if you move the mirror around.
Due to this very 'high-strung' nature, a betta can stress itself out fairly easily. Hence why we don't leave a mirror up against the tank all the time, because the 'fish' they see will never back down every time they advance, and they can become very frustrated over this. The easy way to tell if your betta is stressed or not is to look for horizontal 'stress bars' along the side of the fish. If you see those, he's obviously not happy, and something needs to be changed.
How often are you doing water changes on your tank? Since it's not cycled and doesn't sound very big, you'll need to be doing water changes fairly often to make sure your betta doesn't fall ill from dirty water conditions. I'd go with 50% twice a week as a geneal recomendation since we don't know the size of your tank.