Tell them how you heard of them, what you'd like to see from them and the scope of the work you are looking for them to accomplish. Client lists and references are also important.
1234, Main Street
Boston, MA 02123
08 April, 2005
<Recipient Address Goes Here>
Our company is looking for <public relations firm/advertising agency> to help us <task to perform>. We heard about your firm <where you got their name>.
Could you please call me at <telephone number> to set up a meeting where we can review some of the work you've done for others and how you might be able to help us? I'd also like to understand your basic fee structure. If you have any materials we might review prior to the meeting, I would appreciate seeing them as soon as possible.
I look forward to hearing from you.
encl: <List of enclosed items goes here>