What are some good things to do to prepare for a career as a high school math teacher..?

I am a high school math teacher and because there is such a shortage, you really don't have to go over the top to sell yourself. However, many schools do prefer employees who will be able to contribute to some type of extracurricular activity--a sport, a club, yearbook, drama, volunteering at games, etc

Having a part time job dealing with math will help.

Take every course possible Math Algebra, Trig, Calculus.

My granddaughter teaches High School Math

Volunteer for sure. Also really cultivate a love for numbers, and the magic of manipulating and playing with them. You will be hired if you are enthusiastic about your material. If you can write and publish a math book, even better.