You want to find a publisher who is the right fit for you and your book. A guidebook publisher might not publish poetry; a science fiction publisher might not publish cozy mysteries. So when you’re researching publishers, you want to find a publisher who actively publishes in your genre and will be a good fit for your work.
To find publishers who could be a good fit, you could go to the bookstore or library and look for books that are somewhat like yours—maybe they have a similar subject matter or style. Your librarian will help you. Find out who published those books, and then go online and research that publisher. The Resource Links page also has links to some online publisher databases.
If you think a publisher looks like a good possibility, try to get your hands on a few of their recent books (or ebooks, of course) so you can see them for yourself and get a sense of their professionalism. If you find a book riddled in typos, that might be a red flag, for example.