Welcome friend.
Pick your instructor carefully. The agencies are basically the same, but there are wide varieties in instructors. I am an instructor with the biggest agency, but honestly I see people from all agencies regularly and there is little difference I notice except from those instructors that take the extra time to ensure their students are competent and comfortable in the water.
This is not to scare you, but you often get what you pay for here. Many places offer cheap prices to get people signed up but then don't spend the right time in the most important areas to help you learn the right way.
I recommend that you find out about the instructors personal diving interests. Does he/she only teach or do a lot of personal divign as well. If they only teach, go somewhere else. Instructors that are comitted to diving spend time honing, increasing and bettering their skills, they don't sit idle and hang out in the pool or shallow water all day