Again, depends on all of the above.
If you put the work in, and don't mind spending a little money, anyone can self-pub anything. (But of course, whether anyone will then buy/read it is another question entirely.)
I assume you mean traditional publishing, in which case, being, say, a well-respected professor of archaeology or accomplished venture capital investor is going to make it significantly easier to get your proposal for a book on the politics of ancient Mesopotamia or the hot new technology for 2020, respectively, easier to get picked up than if you're Joe Blow and you want to write a book on either (which is pretty much impossible, barring some other mitigating factor, like being famous for some other reason).
For fiction, it varies widely from genre to genre. But in any case, in most genres you're going to first need to finish your manuscript (a proposal doesn't cut it for first-time fiction authors), then "query" it to agents – after which, if you get picked up by an agent, the MS will go out on submission.
If you're interested in traditional fiction publishing, there's tons and tons of blog posts out there by existing authors, and there's one author in the SFF genre in particular, @DelilahSDawson, who does a series under the hashtag #TenThings, which cover the individual nuances of the process in-depth, but in digestible bite-size threads.
