Simple answer would be to pick up the phone or knock on the door of your local agency. If you need to be pointed in the right direction it would help to know where you are.
Your not the first person to get their EMT cert and then never work a day in the trade but Im curious what motivated you to take the class?
Also there are a few people who have a problem with volunteer services so dont be suprised if they drop by and chime in. Chances are they wont but better to be aware and not caught off gaurd.