Some of you reading this may be thinking, “Now, wait a minute. That’s not a problem, that’s my dream.” I agree with you that this is an enviable question to have to ask. It tends to arise when a (generally) young writer publishes a piece in a magazine, blog, or thesis anthology and a (generally) young agent sees it and expresses potential interest in representing the writer.
Often, however, a young writer has published a story or essay or poem or two, but does not yet have a manuscript ready to show the agent; and this can cause consternation. You want an agent, after all, but don’t want to wreck the opportunity with this person by sending along unfinished work. (I recommend that you never, ever send work out before you and your two best readers (at minimum) think it’s done. There’s a more detailed explanation of this in When Do I Need an Agent? Or: How Do I Know When My Book Is Done?.) What should you do?
Send the agent a polite note. Express your pleasure at being contacted; state that the work is incomplete; and offer to send a sample of the manuscript when it’s finished. You can specify a time frame for this if you have one; otherwise, “when it’s finished” is a perfectly fine statement of when you intend to send it along.
Agents, like all people, appreciate polite and forthright treatment. If it takes you a year or two to finish your manuscript, don’t worry. The agent will still recall the piece that caused her to reach out to you. She reads a lot, and if something grabbed her by the collar, she’ll remember that. What’s more, she’ll likely recall the good manners and professionalism of your response.
In the meanwhile, and after your manuscript is done (I see no need to do this before then, unless you’re looking for a way to procrastinate while writing), do your due diligence. Learn about the agent and the agency she works for. Find out who she represents and how she’s viewed in the literary world. (I recommend doing this by asking questions of actual people—writers and editors you know, for starters.) If you like what you learn, consider allowing her to represent your work. After all, it’s a stroke of good fortune to find an agent who is well respected in the industry, works for a reputable agency, and really excited about your work.
If, after doing your research, you aren’t certain what you think of this agent’s tastes and reputation, you might still wish to send her a sample when it’s ready. Her commentary on the manuscript may be the factor that makes up your mind. Remember that showing her the work does not obligate you to have her represent you any more than her query to you creates an obligation on her end.