So, first of all, not all historians are academic historians. There are people who have studied something different and are writing and researching about history because they are interested or in relation to their field of study. An interest in history can always be rewarding and at the same time helpful in almost every profession one decides to pursue.

But on to the academic path: The "usual" route to becoming an academic historian is to get you undergrad degree in history and then go to Grad School for either an MA/PhD program (as they do in the US) or get your MA first and then your PhD (as they do in continental Europe and – I believe – the UK).

What you need to be prepared for, even in your undergrad is that history in an academic setting is different from history in a High School setting. What historians do is in essence to engage critically with the past on the basis of empirical evidence and methods and theories. Your undergrad ideally will teach you how do that in the sense that you'll learn to recognize methods and theories as well as the use of empirical evidence. E.g. while High School teaches you what happened and – in the best case scenario – what the current broad consensus is why it happened, undergrad will teach you that there is a multitude of ways to approach a given historical topic, what the most common of these approaches are and how you can dissect and evaluate them yourself.

That also means that you will spend your time reading and thinking critically about that reading a lot. I have found that critically engaging with what you read is not necessarily a skill people are taught in High School history classes, so one thing you need to be prepared for is that you will be taught a new skill set and like with every other skill set, you'll need to learn by doing, which – again – means reading a lot, so that is something to be prepared for.

Your MA and even more so your PhD will be about applying those skills yourself, i.e. taking empirical evidence, applying method and theory and structuring them into a narrative that holds explanatory potential about the past. Which means that you will graduate from reading and writing about that reading (because undergrad is writing a lot of papers about what you have read) to engaging with the empirical primary evidence and writing about that.

The major difference here is that you graduate from assessing and critically engaging with what others have wrote about the past to critically engaging with the past itself through the documentary evidence it has left behind. That can be quite the step and involves a lot more thinking, so to speak, especially since you PhD by its very definition requires you to contribute something novel to the state of research, basically to do something nobody else has done before.

After that there is the whole process of finding employment in a university setting that involves different things depending on where you are. In the US you need to start looking for tenure track positions, where you'll have to show through constant research and satisfying work that you're "worthy" of becoming a professor. Elsewhere, you might need to get into a postdoc project and write your second book (as is in Germany) before you can apply to hearings for a profeesorship, provided there is one open at the time.

Every step of this way involves a lot of work, tests and hurdles, career-wise, emotionally, and financially. And the employment situation is atm not too rosy either. I can recommend checking out our currently running series on Grad School in our weekly Monday Methods thread. It can lead to a bit of depressing read but it is important advice that is given there.

Know, that I am not telling you this to discourage you, more historians and people interested in history is always awesome. But if you want my personal advice, don't go to uni undergrad with the aim of becoming a history professor. Go to uni to study history in undergrad to try out if it is something that interests you, that you want to do and that you want to pursue further. I can tell you what the work of a historian entails at various stages up to a PhD but the best thing is to try it out yourself. Getting a BA in history will still enable you to switch fields and careers. A lot of people with a BA or similar in history go on to be lawyers, journalists, employed by state agencies and various firms in the private sector. If history is a field that interests you strongly, go and try it out if it is for you. Because it takes an immense interest and fun in your subject and a whole lot of determination to make it through the process of becoming a history professor or many of the preceding steps. And thus, it is critically important to first go and look what it is that you are getting yourself into.

In short, after you major in history in college and get good grades, you go to grad school to get a PhD if you want to be a history professor/professional historian. It's certainly fun to get paid to read and write all day, but there are lots of downsides too.

Rather than write a whole big long comment here, I'd encourage you to have a look at the series that just started in the Monday Methods threads about grad school in history. Here's the first installment, which should answer a lot of your questions about jobs etc. and keep an eye out for further installments for the next few weeks. While you're still in high school, this is all very far into the future for you, but it'll give you a realistic idea of whether the reality of historical research is something you want to pursue. Also, feel free to ask any follow-up questions in that thread too.