Hehe. Right on about the ALL CAPS, Holly.
When you start out a betta tank, you need to do frequent water changes.
I'm guessing you haven't heard of the nitrogen cycle... and cycling a tank. Look this up online. www.thekrib.com
is a great resource.
Your betta probably isn't eating because ammonia levels in your tank have increased hugely since you put the betta in there (the fish breathes out ammonia through its gills as a biproduct of biological processes). The beneficial bacteria that changes the ammonia to nitrite and the nitrite to nitrate hasn't had the 2 weeks or so that it requires to cover all the surfaces of the tank.
1) Do you filter the tank? 2) Is the betta starting to lose color/look whitish? 3) Have you done any water changes yet? I'd recommend a 50% water change every day for the next 1.5 weeks. I know it's not a fun thing to do, but it means that ammonia won't kill the fish. Esp. if the betta looks white, or is breathing heavily, it is probably starting to feel the effects of high levels of ammonia in the tank. I'd advise you to buy an ammonia test kit from your LFS
(should be about $8). This'll help you monitor ammonia levels in the tank over the next few weeks. You should aim for a level of 0 parts per million. If this level is maintained for a week or so, your tank is fully cycled.
But as I said before, you really need to read up on this subject if you haven't already. Allowing ammonia levels to increase will certainly lead to the death of your betta. I wish fish stores told customers this before they walked out with a tank and a fish...