I recently decided that I want to become a professor of 20th century history. I am currently moving up into my senior year of highschool. How can I go about making my dream job a reality?

So a few things.

There is no such thing as a professor of 20th century history. That is way to broad. I suggest figuring out where you want to go to college, and then taking a wide range of history classes that way you can survey the field and see what interests you. You’ll need specialities though. So for instance, I have a BA and MA in history. I’ve studied all eras of American history. But my main area of expertise is America during the Revolutionary Era (1754 -1802), but my main concentration is Early American Christianity, but my thesis studied the persecution of Maryland’s Quakers by the Patriot Government during the war. Specialities are important and you’ll slowly have to figure out what you find interesting and what you can research that’s never been researched before.
Find a mentor at your college of choice. Try and find one by the end of your sophomore year in a subject area that you love. Have them challenge you and critique your work. It’ll pay dividends long term.
learn from your mentor about what academic historians (professors) do. You’re required to do research all the time and publish if you’re at a research university. You have tons of other responsibilities, which hopefully are right up your ally. But be aware there are 100 other things professors do than simply teach.
Be aware that even if you graduate with a BA, MA, and Ph.D., you’ll likely not find a tenure tract job for years, if ever. Professor jobs are extremely high in terms of competition, so have a back up plan. Investigate other types of historians, like public historians or academic librarians (who study history).

Let me know if you have other questions!