When do I start focusing on being a teacher and how? What are the steps on being a teacher?

First, be a great student! Great teachers are great students of all subjects! Get good grades not only so you can do well on the tests you must pass to get your teaching license, but also so that you will have knowledge. I am a music educator and every field of study is beneficial, and I still, after years of being a teacher, continue to learn anything and everything! Besides what your current teachers ask you to study, learn things online, in the library, at museums...there are many place to be a student! And for sure, if there is a subject you would like to focus on for secondary level teaching (i.e.:Math, English, Science), learn more than what you are required to do at school!
Subjects I studied which I have found EXTREMELY helpful are: Psychology, parenting and child development (mental and physical). Knowing at what age behaviors can be expected has helped me so much. For example: I had an 8-yr-old piano student who had a difficult time physically playing, as well as concentrating. I was able to discuss what I was seeing with the parents, and I learned that the child had been diagnosed with ADHD and that he had also had some physical inabilities associated with a different learning disability. I was able to quickly adjust my teaching so that the student could learn and be successful.
To get experience teaching, teach. At your age, begin tutoring. At your school there is possibly a tutoring program, or maybe you can ask one of your teachers for opportunities. I was a volunteer Math and English teacher my Junior and Senior years, and I learned a lot while I assisted my peers. I also had a few students outside of school who I tutored and got paid for teaching. I would suggest about $10 for 30-minutes, because you are young and without a degree. In college, you might be able to make $15 for 30 minutes. Do you do childcare? For sure do that so that you have experience with children. Some of these kids might become tutoring clients as well.
Finally, don't teach unless you love learning and teaching! Don't just teach because you are good at a subject. I've worked with others who teach subjects because they are good at it, like Math, but that hasn't made them a great teacher. (Maybe you have even had a teacher that you wondered why they were teaching? The teacher that frowns a lot, or the teacher that doesn't connect to students...?) You can have a favorite subject as a teacher, but not only are great teachers great students, but they can also teach anything they have learned and do it well! Become an expert about teaching, not just your favorite subject.

If you want to start now, you might consider a part-time job or volunteer position as a tutor for younger students. You may be able to find opportunities in your own school, or ask friends and family if they know any other families seeking tutoring. The local library may also have opportunities such as reading to young kids, which you might find rewarding.

In college, you can pursue a degree in education, or you can look at alternative training programs (like Teach For America, Boston Teacher Residency, Urban Teacher Residency) after college. Depending on in what type of school and what grade level you want to teach, the advice may differ as to which path you pursue. Different states also have different licensing requirements for teachers, so you may want to Google your state or other states you might want to teach in to get a sense of their requirements.

One more note: if you go online, you can see the requirements for degrees for any college where you might be interested in studying. You can probably even find a list of all the classes in order that they would like to see you take for your specific degree, along with specific course descriptions for each class!