You need a 4-year degree with a teaching credential to be considered "highly qualified" under the No Child Left Behind (NCLB) act. The teaching credential is usually another 20-40 undergrad units, some of which can be incorporated into (counted as) your B.A./B.S. requirements. The teaching credential coursework, theoretically, teaches one "how to teach" - teaching methods, classroom management, dealing with diverse populations, the educational system, etc. Also a big part of the credential is your "student teaching" - where you go into the classroom and teach under the supervision of a fully credentialed teacher for several weeks. Another part of the certification process is passing the state exam for whatever subjects you intend to teach. If 7th-12th grade, you would have to pass a "single subject" test - be it English, Math, Biology, etc. - whatever subject you were going to teach. If elementary, then I believe you have to pass what is called a "multiple subject" test - less difficult, but covering more areas - basic math, basic English, basic Science, basic history, etc - because you will be teaching your students a little bit of all of these things.
To answer the 2nd post in this thread, if you already have a B.A./B.S. (but no teaching credential), you are qualified to teach, but you are not "highly qualified". These days, 99% of schools are not going to touch you without a credential, because the school then has to send out a letter to all the parents of your students informing them that they are in the classroom with a "non-highly qualified" teacher. The parents, in turn, have the right to pull their student out of the classroom and put their student into a "highly qualified classroom".
The exception to this is when your B.A./B.S. is in a subject that there is a shortage of available teachers. If your local district is desperate for teachers in a given subject (usually only math and the hard sciences - biology, chemistry, and physics), and your degree is in that field, they will hire you if you agree to a contract to teach while obtaining your teaching credential on your own time. They will give you, say, 2 years to complete the 27-unit program or whatever it is, but you MUST be actively pursuing it while you are teaching. That is the only way they can get around the "highly qualified" thing. It used to be different, but this is how it is now under NCLB.