You really should cycle your tank first. That's what the majority of people on here will say. Look up the nitrogen cycle for more information.
Essentially its developing the proper bacteria to deal with the ammonia wastes your fish will produce.
Also most people will tell you that a Betta should have a larger tank, more like a 10-15. I can't say much because I've had them in similar size tanks.
Anyway, if you can't get your hands on any substrate or filter media from an established tank, you'll want to keep up with partial water changes daily (pwc's) for the first few weeks while the bacteria develop. Which brings me to a question of do you have any sort of water testing kits? If you don't you should probably pick some up specifically ammonia, nitrite, and nitrates. I own the API test kit which includes all of these. It cost me about 20.
I'd also ask if your tank is filtered or have a heater? It would be better to at least have a heater to make sure the temperature stays stable. Without a filter you'll have to be on top of water changes also to prevent buildup of bad bacteria.
There are alot of people here who can give you some really good advice, and know a lot more about betta's than me. I'm sure they'll chime in.