Go to www.padi.com
This is a nationally recognized organization for scuba divers.
Find a dive center near you and call them to sign up for a open water class.
You can do the book portion online now if you would like.
After you learn what to do, you practice at a local pool.
Lastly you take an open water check out dive. Then your certified.
You can take more classes and get certified in more areas later on.