The requirements will vary slightly from state to state - your best bet is to check the Department of Education website for your state and see what their requirements are. If you go to college in a state other than the one you intend to teach in, be very careful - teaching certification does not automatically transfer from state to state, and you may have to take additional classes where you want to teach if you are certified in another state. Be careful about what out of state universities tell you - my cousing went to a university that told her that her certification would transfer to her home state, and it didn't; she had to take additional classes and recertify before she could get a job. She majored in Early Childhood Education, and has been teaching kindergarden for 6 or 7 years, and loves it. Personally, I prefer older kids - I teach middle school - but if I moved to another state I would have similar problems transferring my certification.
In the meantime, check with local schools about opportunities to volunteer in elementary classrooms. My education program (granted, this was 17 years ago) required 50 hours working with unrelated children under 12, in groups of 3 or larger, as one of the entrance requirements. It could be paid or unpaid. Check with your high school guidance counselor about any programs that are available between your high school and the elementary schools that feed into it - that would be the easiest way to get in. If that doesn't work, go to the elementaries and ask to speak to the guidance counselor (if they have one) or the assistant principal (if they have one) or the principal about how you can volunteer at the school. Check for job openings at day camps, before and after school programs, and daycares - any of those will give you experience with your desired age group.
As far as classes go, make sure you take all of the classes required for college entrance, to give you a good grounding for college. Good grades, extracurricular activities, and good scores on the SAT or ACT will all help you get into college.
Good luck!