Most states today require a teacher to obtain a bachelor's degree in teaching, and then to go through an approved teacher development program, which is principally a year spent as a student teacher. Some states require their teachers to hold master's degrees, and all states require some degree of preparation after graduating with a bachelor's degree. The certification requirements can be filled with layered demands that begin with a baccalaureate degree, but then fold in requirements for additional study beyond student teaching.
At the secondary school level and in middle school teachers stick to one subject - math, chemistry, history, and their undergraduate work varies accordingly. In general teaching requirements are no longer so clear cut and they vary from state to state. If you want to teach somewhere in the Kindergarten through high school range (K-12) you should consult the certification requirements for the state where you wish to teach. Generally they can be found online at the state's Department of Education website.
While it is true that states are the certifying bodies for teaching credentials, there are many instances where school districts have sought and received permission to bend the requirements a little in order to fill their teaching ranks. There has been a chronic shortage of teachers for years; for that reason many of the states have softened their requirements somewhat and will work with districts that have especially acute problems. Some states have special programs for people who already hold a bachelor's degree but in some subject other than teaching.
Math and science teachers seem particularly scarce in many districts, as do special education teachers. Many school districts have been whipsawed by reduced funding from the state and increased mandates from the No Child Left Behind Act, as well as absolute ceilings on class size imposed by statewide statute.
You can obtain a bachelor's degree in education for K-12 teaching from most colleges and universities. Many online schools offer teaching degrees as well, but in most cases they cannot provide complete preparation for the credentialing process because in all states there is a requirement for student teaching hours in an elementary, middle or high school. Some schools known for their distance learning degrees also operate campuses in many states, and some of those have arrangements for student teaching hours with nearby schools.
If you want to teach, begin by determining what the requirements are for your state. If you have previous college credits determine how many would be transferrable to your pursuit of a teaching degree. Many states also require a certain number of credit hours in specific subjects from their middle school and high school teachers; if that's where you wish to end up you need to study credentialing requirements for that reason as well.
If this is for you, do your research and get underway. If you have work hours or children that are a concern, get in touch with an accredited online college about taking the classroom requirements via distance learning, and then ask the same questions of the state department of education or the personnel office in your local school district. Today there is surely a way to obtain a teaching credential without devoting your entire life to four years of on-campus study.