If you followed Step 1, then you have a list of 10 agents that take on your genre of work. Now you're ready to do some more research.
Buy a small notebook to help you keep track of everything. This notebook will also come in handy to keep track of who you submit to later on.
First, check each agency site to make sure that agent is still there and that they are currently taking on new clients. Some agencies only take submissions during certain times.
Check their submission guidelines so you know exactly what they want you to send (just a query, or perhaps a chapter and a brief synopsis). If an agency accepts only queries via e-mail, but the first chapter via snail mail, it is an obvious choice to fork out the postage (since your work has a better chance of being read).
Now you are ready to create a killer query letter.