You should try, as early as possible to obtain some experience working with groups of children or adolescents. Volunteer to lead a youth group, teach Sunday School, coach a team, assist in a classroom, etc. This will not only help you decide if teaching is a good career choice for you, but will also give some of the experience you will need to do it well. Participation in extra-curricular activities in high school and university will also assist you if you want to be a teacher. Drama, music, public speaking and sports activities are particularly useful.
Experience working or volunteering with exceptional children or adolescents is very valuable.