Salaries range dramatically by institution, location, and type of job, and the pay scales can vary over the course of your career (generally upwards) but academic historians are typically paid less than social scientists but a little more than other humanists (e.g., English professors or artists). Which is to say, you don't get into this work for the money. If you get on the tenure track at a not-too-cash-strapped institution then it can be a decent middle-class salary. Here's a table that gives some benchmarks from a few years ago — again, all numbers should be taken with a grain of salt because they can vary quite a bit depending on the place you're at. Keep also in mind the fact that those salary figures also somewhat obscure the fact that most academics in the US at this point are adjunct labor — their employment is temporary and focused entirely on teaching (not research), as opposed to the tenure track (in which research is a large component of the work and, after tenure is granted, the job is essentially permanent).

As for becoming one, it's not as easy as just deciding to do it. You need to go to graduate school, write a PhD, and then get extraordinarily lucky on a exceptionally terrible job market. There are way more qualified people with PhDs in any given field of history than there are jobs. That doesn't mean it can't be done, but one needs to have a healthy appreciation of the odds — it is less like going into being an accountant, and more like training to be a ballerina. Talent and skill are necessary but not sufficient; maybe you'll be one of the lucky ones, but you need to plan as if that isn't the case. Which is a hard pill to swallow, since the opportunity cost of getting a PhD is very high (at least 5 years out of the workforce, perhaps even more, and outside of the academy a PhD in history is a strange thing to have, and can make getting non-academic jobs difficult).

I like my job a lot, but it's important to recognize that I got it largely because I was extremely lucky — the right job opened up at the right time in the right place and they (miraculously) thought I was the right candidate. That's a lot of things that have to line up right (and jobs only basically open up once a year, so if one job season doesn't work out, you have to wait another year for the next one).

There are many other uses of a history degree outside of academic life, and there are even some uses for a history PhD outside of the professorship, but it is not a royal road to riches or even job security. My basic advice to prospective graduate students in history is that if you can imagine yourself doing anything else in the world and being happy with it, give those prospects very serious thought. I know a lot of people whose experiences with the academic job market left them seriously burnt out and jaded, and many of those people went to the absolutely top schools for their degrees.

Assuming you can land a tenure-track job which I think is what you mean by "historian," probably an assistant professor at an R-1 can expect to make somewhere between 50-80k depending on how rich the school is, private or public, and all that. Obviously the range will be lower for SLACs, R-2s, and institutions viewed as less prestigious. But especially in medieval European history, the market is very tight and jobs are few and far between. If you don't get into a top program, I wouldn't bother.