Best of luck!
1. Think of your career like a marathon, not a race. You will find that your career is a lifetime role and keep an open mind as it changes, develops, and meets/exceeds/fails your expectations and listen to how you feel. You don't have to do one thing forever and remaining flexible is important.
2. If you don't know something, that is okay. If you don't have an answer to something, don't stress. There are so many resources and people you can seek in the next 10 years that can help you figure things out. Have the mentality that, "I don't know, but I will figure it out!"
3. There is no bad place to start. Don't overwhelm yourself with your starting place. As long as you get started you are developing a healthy path.
4. READ! Put your phone down and pick up a book on areas that you are interested in. Reading allows you to learn from others and think about self reflect on what you want to do.
5. Take a deep breath. High school and college may seem very stressful, but they are some of the best learning experiences of your life. Try to minimize stress knowing that your future is bright!
I didn't start studying for the SATs until my senior year. That test is just HARD. I was a little too worried about things that had no bearing on my future as a junior and I didn't think what my self-appointed mentors were telling me was that important. I thought my grades were most important. I didn't have the best in-school, paid mentors to guide me. When I say take my life less seriously, I mean I gave a lot of unnecessary time to my social life. I should have given my future more of my time; taken that more seriously. My friends have privileges that I didn't have. My friends had support from other colleges grad in their families. I had to wing it. I didn't know what to ask. I didn't know who to ask. I wasn't used to asking for help in the first place, so that was a barrier too. That last one took years to learn to overcome. It would have been nice to have overcome that deficit as a junior in high school. Life would have been better is I had learned, as a junior, that there are no stupid questions.