A great place to start your attempt to get published commercially is with Query Tracker. Wait – to be clear – a friend told me about it. I have no financial interest in it. I paid $25 for special membership last year, but not this year.
Query Tracker allows you to find an agent or a publisher in the specific categories you’re interested in. Let’s say you’ve written a thriller that’s also a romance. Query Tracker lists 149 publishers who might be interested in such a book. Your job is to search through them, maybe limit the number.
When you select one publisher or one agent, you get their website, a report on them in Preditors and Editors, their guidelines etc. It’s very helpful information.