It depends on the state, but generally you need a four year college degree such as a BS or a BA, plus education courses necessary to get an elementary level teaching certificate. Many teacher prep programs combine the certificate with the degree. The certificate generally qualifies you to teach up to eighth grade, but again, it depends on your state.
In addition to the above *minimum* requirements, you also need the following (to be good at it):
You need these and more because it's the world's most important job.