If you're a senior in high school thinking about becoming a history professor, then you're planning about 10-20 years in advance. You'll need a bachelors (4-6 years), than many future historians spend 1-5 years paying off school debts and getting some real-world experience, then most do an MA program (2-3 years), then finally they enter a PhD program (5-8 years), after which most work as a post-doc fellow (1-3 years), or an associate professor (3-10 years), before having the opportunity to work as an assistant or full professor of history. There are, of course, historians who complete their BA, MA, and PhD in a total of 9 years, but that doesn't always mean they'll be well-prepared for the job market. Over the past few years, only about 50% of humanities PhDs end up working in academia (i.e. getting a university job). So it can be pretty hard, and it always requires a significant amount of dedication and hard work.

That said, I think it's tremendously rewarding. I got my undergrad double major in history and foreign languages, and after five years out of school, I was itching to get back. My senior thesis had been on post-WWI Germany, but I wanted to make the leap into early medieval history. I got an excellent master's education, where I focused on learning languages, reading a ton of history books, and figuring out how important archaeology was to my field. Now I'm in a PhD program, finally narrowing my field of study, and I've started teaching my own classrooms.

It's a lot of fun, but I also think it's a profound responsibility. I only get a few cumulative hours to work with my students, hopefully preparing them to become more civically-minded, better critical thinkers, working effectively to build a better world. I'd love to keep doing this till I'm forced to retire, but if the job market pushes me elsewhere, then I'll be grateful for having had this small opportunity to teach a few undergrads something about themselves and the world around them.

Academia can be quite thankless (especially when your students don't do the reading), but if you're looking for some intrinsic value to your job, teaching history can be a great way to go. Things you can do now or in the near future:

(1) Learn languages! Even American historians are expected to have an extra language or two under their belt. Medievalists or those who work on modern cross-cultural issues often need even more.

(2) Take an upper-division (300- or 400-level) history class your freshman year. It'll be hard and demand a lot about learning how to write and think, but you'll get a quick feel for whether this is right for you. Talk with your academic advisor to pick an appropriate course, and be sure you email the professor (be respectful and use appropriate formalities!) and meet to discuss your goals during the first week of classes.

(3) Keep your options open. If you work too hard at a single goal, undergrad can become rapidly exhausting. It should be an opportunity to really explore who you are and what you're capable of doing. Most importantly, it should be fun! And if you remain determined to become a professional historian, having that wide knowledge base can prove a godsend for years to come.

To add to what /u/textandtrowel said, I would say that if you're in the US think very hard about going down this line. Getting through academia to grab hold of the PhD is hard work, but it's more about hard work than talent. Getting a job as an academic teaching history and not as an adjunct, that is what is truly hard. The US academic jobs market is quite simply very unfavourable, you are most likely not to end up a professor of History, and unless you are already wealthy/exceptionally talented, you will have a considerable student loan burden.

I think history is wonderful, and if you do decide this is what you want to pursue, I wish you all the best. I simply want to point out the grim realities of humanities academia and the job market.