Q - How does one get his or her foot in the door of a publisher if you have never been published? Do you include any sort of resume? I don't have and previous publications so is it all chance when you send your manuscript? Should you always send a query letter before sending a whole manuscript even for small stories or children's stories?
A - I'm not sure if you are referring to books or short pieces for periodicals, so I'll try to answer for both. I would say that for either market the most important thing is to follow the publisher's guidelines exactly. For periodicals, some publishers want a query first--while some only want a complete manuscript. The market guide always tells you which they prefer or require.
For a book publisher, they may want just a query first, or it could be a book proposal, or even a full manuscript. They do want a resume, but rather than everything you've ever done, it should focus on what experience you have that qualifies you to write the article, story, or book you are proposing. For example, if it is that children's story or book, the resume needs to highlight what your recent/current experience is with children. If it's a Bible study they will want to know what your experience is in teaching Bible studies, have you tested the material on an actual class?--that sort of thing.
The bottom line is that following their guidelines will get your foot in the door. Having a stellar manuscript will grab and hold their attention.