The first page of your story has to grab the reader, agent and editor. There has to be something in that first sentence, that first paragraph, that first page that grabs and holds them like a pit bull. Many times in traditional publishing they ask for the first three chapters or the first fifty pages. Polish them well before you send out. Remember you get the story out in the first draft and write the whole book. Then you revise, revise, revise. In that process you will polish the first three chapters to a high gleam so that the agent or editor asks you for the rest of the manuscript.
Send off and Wait or What?
You will not wait for a reply from those agent and editors, but you will begin your next project. The process of hearing back from an agent or editor can take a few days to a few months to a year or more. You need to develop a body of work, so start your next book.