To get on the NY Times Bestseller List you will almost certainly have to do so with a book that is traditionally published.
You become a professional author when you treat writing as a profession, and you produce books that are of professional quality and make them for sale. You can do this via traditional publishing or self publishing, but as mentioned, if your goal is the NY Times Bestseller List, you need to focus on trad.
No, you don’t have to do this and only this for a living. In fact, only a teeny tiny percentage of writers would EVER be able to do this for a living, unless they either have a spouse with a steady job (and insurance) or are willing to live as a starving artist forever.
You educate yourself about the industry, query a professional-quality manuscript, and treat all negotiations and interactions as if they were business interactions – because they are.
Well, this is a question open to debate. Some would say don’t put anything controversial out there, but there are some VERY popular writers who are uber political, use constant bad language, and who are happy to wade into shitstorms of controversy.
Ultimately, I think you need to think about your BRAND. What is appropriate for your brand? If you write sweet romances or children’s books, I’d say dropping f-bombs in every post is inappropriate.
The most important thing to understand, though, is that social media is SOCIAL. It’s not for marketing. People will not follow you and read your posts and share them and interact with you if it’s all about you and your books. Be an interesting person; don’t troll; be generous with your sharing of and interacting with other people’s posts.
Honestly, I think the only thing that’s truly out of bounds is being an asshole. You can e controversial – just don’t be an ass about it.
The million dollar question. Write books people want to read. Define your target reader. Let your target reader know your book exists. Rinse and repeat – a lot. What works to do all that changes frequently, so I’d recommend you find online groups with working writers who are sharing what works and study it (and keep studying it).
People want to read things they believe they will like. They trust the formula. They also trust their friends. Finding people who love your work who will sing your praises to their friends is the secret of great success.
Your subject line, by the way, says “beginning your journey.” Honestly, the first step is to master the craft of novel writing and learn to write a professional-quality manuscript. That can take years and many, many trunked novels. Don’t focus so much on publication that you gloss over the first step.