As I conceive of it, historians can largely be divided into two classes: academic historians and popular historians. A popular historian just requires you to write a book about history, written for a large audience. Many popular history books are written by journalists, but anyone can do this (including tenured professors, such as Barry Strauss or Bart Ehrman). An academic historian requires you to earn a doctorate degree in your field of choice, and then acquire an academic job (which is slim, given the current state of the job market, and is largely relegated to the top 10 schools for each field.)

I am currently a graduate student for Late Antique history, which is very close to medieval studies (I got into this field through my love of medieval history), so I can also offer you some advice for breaking in to medieval history. The single most important thing you can do is to learn your languages. The earlier the better, as it gives you more time to improve as the years go by. It will depend on what kind of medieval history you're interested in, but Latin is going to be essential 99% of the time. I started taking Latin in the 9th grade; if you have the option to do so, I would do the same. My other primary language is Greek, but those doing later medieval history would instead look at the vernaculars of their given region (Old French for France, old English for the English kingdoms, etc.)

In order to get a career as an academic historian, you must get into a good graduate program, which requires you to be excellent in your undergraduate institution. Take classes in your primary languages (I was almost always taking at least one Latin class, as well as learning Greek starting my sophomore year), and learn a modern language (French or German are the most important, but it'll depend on your subfield). Take classes both in your area of interest and in the wider field of history, as well as the complementary fields of religion and literature. Build strong relationships with your faculty; this is not 'networking' but building professional relationships, where they will guide your first steps at becoming a scholar. They will also be the ones who write your Letters of Recommendation for your graduate program. Take the time to learn to write well, and (if you can) develop a piece of original research that you can present at a conference and use as your writing sample.

Finally, cast a wide net when applying to graduate programs, both for an MA and for the PHD. Only apply to places that will give you funding, and don't go into debt for a graduate degree in the humanities. If they pay you, then the worst case scenario is that you've just spent a decade of your life reading, writing, and learning; if you pay, then the worst case scenario is financial ruin with nothing to show for it.

If you have more detailed questions, I will attempt to answer, and feel free to PM me if you'd like.

Hi, OP! I'm a medieval historian.

Don't worry too much about grad school and your college major and all that stuff. You have PLENTY of time for that down the road. You'll get fantastic advice from your undergraduate advisors and your favorite professors when that time comes. And just to ease your mind--I went to a completely ordinary, no-name undergraduate school, and yet I'm doing my PhD with one of the world's best scholars in my subfield. Go to Harvard or Oxford if that's the right school for you, not because it's "necessary."

There are a couple of useful things you can do in the next four years that could prove really helpful, though!

The first thing is to start working on languages. Especially if you want to study the Middle Ages, being good in multiple languages is necessary. People who focus on Western Europe, like me, all know Latin plus English, modern German, and modern French--and then the medieval languages from whatever region we study. Most of that will come in time, of course (I picked up medieval German on the basis of knowing modern, for example, and am working on medieval Dutch through my knowledge of medieval German).

But you can get started with at least one language now, and then add another in undergrad. If you're thinking about the Middle Ages, I'd suggest Latin or German. People generally have an easier time picking up French quickly or through a dedicated summer reading course, than Latin or German.

Second, READ. Just read, read, read. Don't worry about "focus" or being any kind of expert at this point. Read ONLY what it interesting and good (ask us here for recommendations!). You will have all the time in the world to read boring books for school. This is your chance to read for fun and what you love and to discover new things that you love.

You have the amazing chance right now to learn and love without the pressure of grad school admissions, paper deadlines, and trying to find a job in history. Use that time to read, to learn languages, to go to Renfairs, to follow Internet debates about the historical accuracy of different movies and TV shows.

Use it to love history just a little bit more, every day.

Good luck!