Congratulations!

For your professional career as a historian, here's some up front things I wish I had known (and taken seriously) when I started:
- There are professional organizations for historians (especially the AHA, but also the ones pertaining to your subfield). It is important you become a member, learn quickly who is who in your field, and start developing "your" professional network. The reason for this in the end is extremely simple: when the time comes to get your manuscript published, application for a job or scholarship reviewed, etc, the people who will review it either fall into the "Victoria Jenkins? Never heard' o her. What does she do?" or the "Victoria Jenkins? Oh, I seen her at the AHA; gave a lovely paper on the rise of Shindigism in South Eastern Sumption, she seemed smart." categories. People who have, even if only once, seen you in a professional context, are more likely to consider you in a professional context. It's simple, but too easily forgotten. So find out what the professional organization networks are in your sub field. Join up as soon as you can.
- Read book reviews. Then, as quickly as you can, volunteer to write them (e.g. for H-Net, or the journals specializing in your field). That's another way to get your name out there. No, it doesn't pay money. Yes, it does pay back in training you for your field. Be fair, concise, critical, and honest in them. Nothing else matters when it comes to book reviews. Do not brown-nose, but not be unduly harsh: it's a lot of hard work to get anything peer reviewed and published. Treat your peers with the fairness and honesty that hard work and history deserve. A well-written review will get you a good reputation. It is definitely worth your time.
- Partake in graduate student conferences. Yes, even with just a B.A. you can and should get started in this. Take your own work seriously. Present papers.
- In "professional grade history", it matters as much what your peers are discussing than what you are interested in. You might be more interested in the rise of French pre-revolutionary exorcism, but the current academic focus might veer towards the debate on fighting witch superstition in post revolutionary France. Take that into account. Know what the current scholarly debates are about, and be ready to engage in them. There is a reason why your field is currently focussing on something and not other. That does not mean you shouldn't follow your instincts and interests. (Some of the greatest successes in historical writing were in books on subjects suddenly coming completely out of left field, landing on the market just when academic focus randomly was switching there.) No, one cannot predict this, so yes, follow your interest. But be aware what your colleagues do. Is there a pattern? Why? If you want your writing to be taken into account, you must contribute to the current debate and engage your fellow historians' points, not only attempting to start your own.
- When writing, never forget the golden laws of Hugh Trevor-Roper.

Finally, don't ever forget that you must do what makes you happy. I've seen a lot of people who were actually unhappy in academia, and they couldn't figure out why. I've also seen many very happy people there. But sometimes people find out that living a life of books is not actually for them. Do not hesitate to make big changes if the need arises.

I see other commenters have given you very good advice about the practical aspects of being a grad student, so I'll try to give you the career advice I wish someone had given me when I started grad school: know what you want to do when you finish, but don't expect it to work out and have a plan in place in case it doesn't.

I'm going to level with you: the job market for history Ph.D.s is absolutely abysmal right now. The traditional academic career path of getting a Ph.D. and finding a tenure-track job, for all practical purposes, does not exist anymore. The number of tenure-track jobs available is shrinking every year, and even when tenured professors retire, most places are replacing them with hordes of non-tenure instructors and adjuncts. Even museum and archival jobs are getting harder to come by than they used to be; I needed a PhD and publications to get an entry-level, subsistence-wage museum job five years ago, and it's only gotten worse since, especially with COVID-induced hiring freezes. Too many older professors are selling their idealistic young students on the idea of a future that just doesn't exist anymore.

Now that I've got the harsh truth out of the way, I want to give the practical advice: do everything you can to make yourself as broadly qualified as possible. Not necessarily in terms of field, but in terms of skills. Obviously, languages are going to be a key skill for you, so the sooner you start working on those skills the better, but start thinking about other ways you can expand your skillset as well. One of the few real growth areas in the humanities right now is digital humanities. Academic institutions (including museums) have really fetishized the digital humanities recently, so if you have skills in things like creating and managing databases, you're going to have a lot more opportunities available to you. Also consider how the skills you're developing might be useful outside of an academic context, because, frankly, there's a good chance you'll end up with a non-academic job whether you want to or not. The more skills you have and the more broadly-applicable those skills are, the better your odds of gainful employment after graduation are. (Also, while you're still a student, think broadly about what funding avenues might be available to you for research, etc., and ask around for more advice. You can never have too many funding options, and fellowships can be a valuable source of networking, as I'll note below.)

Another thing I can say from my own experiences is that you cannot neglect networking. And I don't just mean schmoozing at conferences, I mean every random little interaction you have with a fellow academic. I'll give you my career trajectory as an example. In 2015, I was finishing the research for my dissertation and had a few chapters written. I was at [Museum] doing some work with their collections. I took the elevator down from the library to get some lunch and happened to be in the elevator with an editor of [Project], who asked me what I was working on. By complete chance, they happened to be working on a volume of [Project] that related to my research, and I was asked to make some contributions to [Project]. I wrote the essays they wanted, which netted me maybe $200, and I didn't think much else of it. A few months later, I was at a workshop organized by [Fellowship] at [Museum]. I ended up at a dinner table with the head editor of [Project], and talked with him about my research and the essays I had written for [Project]. Fast forward a year, a month after I've gotten my Ph.D. and don't have a damn thing to show for it. No job offers, exactly one interview out of over a hundred applications. I was trawling H-Net in my usual state of half-apathy, half-desperation, when I see that the head editor of [Project] has posted a job opening for a position at [Museum], working on [Project]. I apply, noting that I've contributed to [Project], and hoping that he remembered that dinner conversation a year earlier. And that's how I kept myself out of the unemployment line: a random thirty-second elevator conversation and a chance meeting at dinner.

So, yeah. Begin with the end in mind, and don't kid yourself about what you're up against in terms of the job market. I would love to give you some idealistic Disney-movie speech about chasing your dreams, but that's not the reality you're entering. The people at the top pulled the ladder up behind themselves decades ago, so keep an open mind about what avenues might be available to you with the skills you're developing. Go ahead and start checking academic job postings (H-Net, Academic Jobs Wiki, USAJobs, etc.), not to apply, but to see what skills and qualifications they're looking for, so that you know what skills you need to develop. You'll have a much better chance of success if you start with an idea of what you need to be working toward, rather than just going through the usual process and hoping there's a light at the end of the tunnel. I'm sorry if I came across as mean or cynical in this response, but this is the truth that people considering graduate studies (especially in the humanities) need to hear, and they're not going to get it from their old, comfortably tenured professors.

If you have questions, I'd be happy to answer them. Good luck.

A master's can be a lot of fun. In the programmes I am familiar with, you get to take several different courses, and do a lot of reading of things you probably haven't read or heard about before. I found, when I did my masters programmes, that it was the time in my schooling where I could look back every Friday and say, "wow, I learned all of that in just the last week!"