Find a school that will award you a Bachelor's degree in teaching Spanish. There are literally hundreds of schools, so you can find one in your state or go somewhere else. Look at college handbooks, talk to your current Spanish teacher or guidance counselor, or call schools in your area to ask about their degrees. Make sure that you choose one that not only has a Spanish program, but a teaching Spanish degree.
Speak to the school of your choice to find out if you will be studying Education with an emphasis in Spanish, or if you will be studying Spanish and obtaining a teaching certificate. Hundreds of schools offer a degree, and about half of them offer each type of degree. If your school requires you to get an education degree with an emphasis in Spanish, you'll be getting a full, K-12 teaching degree, and taking Spanish courses on the side. However, if your school is the other way around, you will be focusing on learning Spanish, and taking just enough education courses to get your teaching certificate. All schools are different.
Apply for and enroll in the school of your choice. Meet with your adviser, register for classes, buy books, and begin to go to class. Depending on your degree program, you can expect to attend classes for three to four years.
Complete your school's requirements for student teaching in the last semester of your degree program. All schools have different requirements, and depending on your degree, you'll spend anywhere from a year to a quarter teaching in the real world under the tutor of an actual Spanish teacher.
Apply for Spanish teaching jobs once you have obtained your degree.