Morning Severian, as a first step, get a copy of the Writers' & Artists' Yearbook - it's really helpful in terms of process. So, first thing: publishers do not read manuscripts from writers - it takes a lot of time, it costs them money. They've more or less outsourced that work to agents. Agents will take a 15% cut (which given how tiny book advances are unless you're a celebrity is a pittance) and they're the people who speak to the editors at publishers - finding one with the right focus makes a real difference. The agent will employ freelance readers who will advise them if your proposal is worth a shot - the proposal is the key; it has to be exactly as they ask on their website or they'll usually turn it down because it means they can't trust someone to respect the relationship.
As part of a proposal, one section is the 'biography' where you, briefly, describe why you're the person to write this book, what it is about you that gives you the credibility - so, yeah, a background in journalism helps for sure. And don't be shy about making the most of your experience (not lying but don't hide either.) In my case, I have a normal day job, I work a full day, I write in the evenings - this whole Thurston book was done between spring of 2015 and summer of 2016 entirely at night. Showing a record of writing (self-publishing, etc.) can help; showing that you have an expertise and an audience in a field (in my case, the Nirvana-Legacy blog); social media presence (i.e., FB, Twitter, etc. with a fair follower number, etc.) coming to them with a solid and exciting proposal; being able to show them you have most of the work done so it's a fairly safe bet... It helps.