Word of caution. Never send a full manuscript to a literary agent unless that person has asked you to specifically do so. First send a query letter, but there are a number of steps you’ll want to take before you query anybody.
Start with some serious research. Make a list of ten or twenty agents who seem to be a fit for your material. Then study up on each one. Does the target agent(s) handle subjects or categories similar to yours? Visit their website. What titles have they represented? Is your manuscript too similar to a book the agent just recently represented. If so, they probably won’t be interested in your work. Can you find out anything about what the agent likes or doesn’t like, professionally? Has that person done any articles you can read or given any speeches that might give you some clues.
Next, find out exactly how each agent likes to be queried. Agents usually have submission guidelines on their sites. Follow them! Some agents require a query letter plus three chapters. Others want only a query letter and if interested they’ll request more.
Now that you have narrowed down your list, send the query to all of them. Be sure to mention in your query that you are making multiple submissions. Then wait. Don’t contact the agent to ask if they received your query. Don’t ask them why they haven’t answered you yet. Just wait at least 90 days. If you do not receive a response during that time, the targeted agents are most likely not interested.
Start the process all over again with a new list of prospects.
Don’t be discouraged. Often, writers query fifty or more agents before receiving a response.
Meanwhile, keep writing.
And never give up.