It depends on how you define "historian". At its most basic, a historian is someone who increases our collective knowledge about the past - no one needs permission to do this. You can head down to your local archives tomorrow, and start looking at historical documents and artefacts to learn about the past. Write about it, get someone to publish it (or hey, make a website and publish it yourself!), and you're a historian.

In terms of historical careers, that's a little different. One option is doing the above, but doing it well enough and interestingly enough that you can convince people to pay you to do it, through publishing books, making documentaries, podcasts, YouTube videos or whatever else you can monetise. In most people's mind, though, historians tend to work at universities or other knowledge-based institutions (libraries, archives, museums and so on). Most such roles require postgraduate qualifications, which in turn requires a relevant Bachelors degree. Depending on where you live, this is usually a long, difficult and often expensive process, and securing a place (let alone funding or scholarships to pay for it or your living costs) can be very competitive. Even then, in the current job market, there is no guarantee it will ever actually lead to specialist work in your field - there are simply far more (over)qualified people than there are jobs at the moment.

My point here isn't to try to scare you off trying, but to encourage you to reflect on the ways in which you want to be a historian. If you love history, there are ways to express and develop that other than through your career - you can read and research in your spare time as a hobby, you can volunteer at local museums or as a tour guide. Hell, you can research and write answers for this site, and reach a bigger audience than many professional historians. But if you do decide you want to pursue history as a career, then you need to go into it being aware that it's going to be long and difficult, with no guarantees that there will be a happy ending no matter how hard you work.