Depends - How big is it exactly? One shouldn't compare their own tank to the petstore-cup as the cups aren't ever permanent to begin with.
The smaller the "tank", the more water changes you will have to do. Betta keeping CAN be easy, but at the start it takes some work to make it easy in the long run.
The reason you need to do more water changes in a smaller tank is because of the buildup of ammonia. Ammonia comes from rotting fish food, and fish poop/urine. As you can imagine, a small tank will easily fill up with ammonia rather than a large tank (given the fish are the same). Oh, and ammonia is VERY toxic to fish in high levels. It can negatively effect them in the long run, or kill them (which is very common when fish keepers stick a bunch of fish together and the ammonia levels SPIKE UP because of all the fish at one time). Ammonia is never good for the fish.
In order to KEEP those levels down, you "cycle" the tank (this is not a filter. Technically it's the Aquarium Nitrogen Cycle). It's not easy to learn at first, but with time it gets very simple. Here's a starting place:
Ask any questions you need to, don't be shy. :)