“Get started on learning a language.”
First, I recommend you consider taking much foreign language. Of course Spanish can be used much in both North America and South America. French is the trade language of much of West Africa and North Africa where many Muslim unreached peoples live. Talk with other students and your school counselor and find what language teachers are the most effective teachers and consider taking the language they teach. Getting a vision how you can learn a language is even more important than which language you pick.
Second, as a junior or senior in high school, take the Perspectives on the Perspectives on the World Christian Movement course for college credit. This is available one night a week, usually in the spring. My son did this as high school senior and a few years later, my daughter as well.
Most of all, I encourage you to talk and pray with your youth pastor about all of this.
Answer from John, a long-time mission mobilizer.
“Become a global Christian.”
Look for opportunities to build relationships with people who are culturally different from you. You may be able to volunteer right in your own city with immigrants or refugees, helping them with English, and perhaps assisting them with their settlement.
Make friends at school who are from outside the USA, or who come from immigrant families. Ask gentle questions about life and how they see it. Rather than working to make them more American, see if you can understand their home culture.
Take any school course that exposes you to the world outside your own country: international relations, international business, world history and geography. Be knowledgeable. And read, read, read biographies and books by believers who have lived in other countries.
Answer from Elizabeth in Michigan who has served with SEND International all over Asia for 40 years.