If you don't know any other writers you might be able to find some at a conference or through people you know. Librarians, bookstore staff, and English teachers might be able to help you, but just telling as many people as possible that you want to meet other writers should lead to some introductions.
You'll need to agree on a meeting time and place if you will be meeting in person, or decide if you want to establish a Yahoo group or just e-mail each other directly if you're doing it online. Then you can discuss the things mentioned in my last few posts and agree on how the critiques will be handled. Although it's not a major factor, for face-to-face groups you might also want to decide if refreshments will be served and how they will be provided if you have them. Some other things to consider are the genres to be critiqued and the maximum number of members to be allowed.
It may take a while for a new group to get established, but once it does you'll probably not only help each other with your writing, but become good friends as well.