Who to send to? Who accepts what? Who's looking for what genre? Who is currently looking for new clients?
If you're on the hunt for an agent, there are two great sites out there. The first one is www.agentquery.com/
There, you'll find tips, resources and advice on submitting to an agent, plus it has an advanced search feature to help you find an agent targeted to your specific genre. This one also allows you to find out what agents represent other authors.
The other site is www.litmatch.net/
This site has similar features as the first site, and this one also has a "submission tracker" feature to help you stay organize. Isn't that nice? :0) The results are a little easier to follow from this site.
Both sites are great and allow you to find agents who are actively seeking new clients and will also tell you if they accept e-mail queries and what works they represent.
Good luck in your search