Welcome to the forum. Feel free to jump in any thread that catches your interest.
Yup as Michael said.
Simply comment on blogs and threads as they catch your interest, Join some groups, invite you atheist friends and so on.
You could probably make a post about why you are an atheist (That's what I did). Or come up with thought provoking topics for discussion. I'm sure everyone here is open to your thoughts.
Hope that helps!