Once you have completed your novel (keyword being completed) it is time to write the query letter. Basically, be professional and keep what you have to say short, to the point, and error free. Below are some articles with more in-depth advice.
“How to Write a Query Letter” by AgentQuery.com: http://agentquery.com/writer_hq.aspx
“The Complete Guide to Query Letters” by Jane Friedman: https://janefriedman.com/query-letters/
“How to Write the Perfect Query Letter” by Mary Kole: http://www.writersdigest.com/online-editor/how-to-write-the-perfect-query-letter