Steps for Obtaining Certification
You must earn a professional license to practice teaching, just as you would to practice law or medicine. Everyday you lead a classroom, your actions will influence the life trajectory of a group of students - their brain development, their characters, their aspirations, and their self beliefs. Given this incredible responsibility, states have put into place a process to make sure that anyone who assumes this role has the necessary skills and expertise.
The professional licensure requirements are set at the state level, so there are variations in policies across states. Thankfully, a few things at the big picture level are fairly consistent, and as an aspiring teacher, you do not need to know all the intricate details and nuances of each license in each state. Just the state(s) where you think you might want to teach.