If you want to be a traditional academic historian, you become a historian by getting a PhD in history. Getting a PhD in history will take at least 6 years. Some people get a masters in history before moving onto their PhD, some people go straight from undergrad. Depending on your subfield, there are normally a certain number of languages you need to know-- for ancient history, for example, you normally need reading proficiency (which is very different from fluency- it's a much lower bar, but you need to be able to read in any language you research in) in latin, ancient greek, french, and german. A good PhD program will generally be fully funded, through a variety of fellowships and teaching assistantships-- it is absolutely never worth paying to get a PhD in history. Those good, funded programs, are often very competitive, and require more than good grades and a good GRE: you need a stellar writing sample, original and directed ideas (even though your dissertation topic will probably change), and a member of the faculty willing to work with you. Getting a PhD in history qualifies you only for becoming a professor of history, a profession for which there are practically zero good jobs (the only well paid jobs in the field are tenure track positions, which offer job security and benefits. Post-docs and adjunct positions are generally underpaid, overworked, and do not offer benefits.) It is the opinion of most people on this sub that getting a PhD in history is absolutely not worth the time and money. There's this iconic thread about it by u/commiespaceinvader.

Getting a PhD means giving up on 6 years of a salary and work experience in exchange for between 15-35,000/a year, depending on the school, with few prospects and little training to succeed outside of the academy. Some people (myself included) try to do it anyway. A good idea? Probably not, but if you can see yourself doing absolutely nothing else (seriously, nothing else), you love research, and you have ideas for original contributions to the field, it might be worth considering. I don't think it's terribly productive to tell people, as a blanket statement, not to do something, but I think choosing to become a traditional "historian" is rarely the best decision and takes a lot of careful thought and consideration.

There are other ways to do history that don't involve being an academic historian. There's the field of public history, where having a masters or bachelors is more the norm. Public history is less about doing research on niche academic topics, and more about finding out how to best tell that story to the public. There's archivists, who normally have a MLIS or some adjacent field, who focus on processing and cataloguing records, preservationists, who often have chemistry/science backgrounds and relevant graduate work. There are also writers and journalists who write history books, though they're rarely the best researched works. Becoming a traditional historian is a huge investment with often little reward, but there are certainly other jobs that are adjacent to the field of academic history that don't require the same depth of training.