You write You rewrite. And you keep writing. Never give up. It helps to have innate talent, but writing can be learned. You will face a fair amount of competition because writing is somewhat of a glamour career. Lots of people want to write and regard themselves as writers. After all everyone is taught to write in school.
It was not that hard to support yourself in the field when I entered it in 1977. But lots has changed. It's tougher now. The internet has brought global competition wiling to work for less than a living wage in the U.S.
Your best bets are marketing and public relations or some variation of them. While the digital publishing revolution had made it possible for everyone to publish a book, it's quite another thing to sell it. It's actually gotten harder to sell ebooks in the last several years as more people have flooded the market with their creations.
I've watched many fellow authors try to shore up their bottom lines by editing other people's stuff, mentoring other authors, and more. Others have taken full-time jobs. Nothing is impossible, but to succeed as an author you need to be a skilled promoter in addition to being a skilled writer.
Because the media no longer has the advertising base it once enjoyed, journalism is a diminishing career. It may be wise to have a backup career; some like to hire medical professionals for write about medicine, for example. Still, there will be room for talented, skilled and objective journalists willing to work in the field.
Well, my question is, why are you interested in becoming a successful writer? Do you have a story to tell? Do you want to do something that's creative? Success in writing, or virtually anything comes from hard work, looking for the right places to do you writing...and to just...write.
Remember that there are many different forms of writing. I have written TV shows, a film, many animation series, comics and graphic novels, non-fiction books, and videogames. I still want to write a novel, but haven't done that yet. Other areas of writing include advertising/promotion, journalism, poetry, technical writing (could be in various areas such as computer science, medicine, science, etc.).
So research the various types of writing to get a feel for what you'd like to pursue.
In general, read a lot. Study the area of writing that interests you. Then write, write, write, and keep writing and rewriting and keep at it. There is no shortcut. Writing is work and it's also a craft. Like any craft, it takes training and practice and persistence.
Finally, define what you mean by "successful". Is being published/hired enough? Do you want to win awards or are you happy to put out good work? Personal passion is what drives most writers.
Do you think making a certain amount of money = success? If so, I would rethink being a writer.
It largely depends on how you define success, and for what kind of writing. Luckily, there are plenty of opportunities and avenues for success in writing. Regardless of the kind of writing, the way to be successful can be boiled down to learning and practicing all kinds of writing. Being well-versed in grammar rules and how to edit thoroughly. Understanding mood and tone, and appropriate symbolism.
Writing creatively you might find strengths in narratives, character building, and endurance (as you'll have to write *a lot*) and you might judge success by getting published in a community/school zine, or publishing a short collection of poetry, or publishing a novel or book.
If you're more of a technical or professional writer you may be stronger or have additional skills in catering to your audience, clarity, concision, or perhaps procedural writing. You might judge success by the number of clients you have projects with, publishing a work of non-fiction, or helping edit a book that gets published.
For me, I'm a UX writer and success for me is working at an excellent company with room to grow, helping design software products, getting to do user research, and making reasonable money. I found success by writing and reading a lot, and practicing both creative and technical writing. Further success would be authoring a book about the craft of UX writing. Dream big, and put in the work to read and write as much as possible.