Here is the deal with bettas.. They were originally bred to fight. That means people bred them to be aggressive--even the bettas now are aggressive. So in general, they don't often do well with other fish, even if the other fish is also a warm water small fish. Sometimes they do fine, sometimes they don't. For a first betta it's usually best to house them alone. Plus you need a 10 gallon tank to house a betta with any other fish!
They like living by themselves. :) Imagine being trapped in a small room with some stranger--all the time!! It's stressful. Either the betta nips the other fish, or the other fish will nip the betta! Again there are exceptions, but in general these things can and will happen! You can look at the compatibility section if you must, but be aware of the tank needs, your betta needs, and all the personalities. And the more fish, the more you have to change the water.