I am glad to hear you are interested in possibly pursuing a career in teaching. I taught in high school classrooms for several years, and it could not have been more rewarding. First, I will break down the specifics needed to pursuit these careers, and second, I will offer general advice about teaching.
My first piece of advice is to decide on whether you want to pursue a career as a professor or as a K-12 teacher (it sounds as if you are specifically interested in secondary). Professors and teachers have two separate career paths. College professors require a graduate school degree in a specific subject area. Generally, these degrees come in the form of a Masters degree (generally 2 years) or a PhD (generally 4-7 years). For example, if you were interested in becoming a History professor at a community college you would at least need a Masters degree in History. Many university programs, however, require that you have a PhD in the subject area.
It sounds like your undergraduate school has provided you with the option of obtaining a teaching credential along with your degree. Teaching K-12 in the United States requires a teaching credential. If your major's program does not provide a teaching credential, then you will need to enroll in a credential program at an accredited school. These credentials are state specific, as different states have different requirements. I would recommend looking at your state's specific teaching website to determine what credential programs satisfy this requirement. Along with a credential (which again, it seems as if your college has already made available to you), states require you to pass state specific exams to teach your desired subject. So, if you are interested in teaching secondary English, you will need to take state specific English subject tests. In California, for example, these subject tests are known as the CSET.
My advice towards pursuing a career as a professor would be to be passionate about your area of study. Graduate school requires an incredible amount of time and energy devoted to very specific subjects, and after graduate school, professors spend their careers continuing to research and teach their subject area. If it is the specific subject matter that drives and excites you, whether its biology, history, or literature, consider looking more into a career as a professor. I would recommend submitting a question to CareerVillage with the specific subject area that you are interested in teaching as a Professor or asking a graduate student in your field of interest at your university about applying to a graduate program.
In comparison, if you are drawn to teaching because you are passionate about the classroom and driven by impacting young learners, I would strongly consider a career as a teacher. The classroom is a unique place and I would recommend getting into the classroom as often as you can. Whether it's through shadowing a teacher, classroom observations, student teaching through your credential program, or substitute teaching, it is important to gain familiarity with the classroom. Teaching is not about getting everything right or being mistake free, rather it is about being comfortable in a diverse and ever changing environment, and being able to adapt to best serve your learners.
Finally, this advice is not to say that as a professor you do not impact learners and as a K-12 teacher you should not be passionate about the subject. However, there are very different requirements for teaching at a secondary school versus at a university or college, and your decision should be driven by what you will find must fulfilling in your career.