I work in the school, so that makes things easier.
Do you have the interest/support of the HT? What would you say would be the aims of your group? When would you run it? What would be the optimum age range and size of the group?
When we started our clubs (2.5 years ago) I worked with another teacher who was also passionate about reading. In secondary reading is often viewed as being not cool, so we felt we had to do a heavy marketing job to launch the club.
We identified some potential members and send invitations home, but also put posters up round school stating all were welcome.
We also have various initiatives to encourage students to attend - merit points for regular attenders, certificates if they come every week for a term, parties at the end of term, etc.
We rarely do the 'all reading one book' model, because we have such a wide variety of readers. In my current yr7 club I have students with reading ages from 7 to 16+!
Our aim is to encourage them in their reading, to get them to become lifelong lovers of books (if they are not already) and to get them to read a wider range of authors and genres.
We do a wide variety of activities such as writing to authors, quizzes, puzzles, reading the opening of a book together and discussing what we like/don't like, questionnaires about their reading, writing competitions, watching a DVD adaptation of a book, having a visitor in (journalist or bookshop owner or librarian), silly games, designing a new book cover, etc. etc. Generally students do not enjoy writing book reviews, so we only ever do this as an optional activity.
We only have 45 mins and during this they also need to eat their packed lunch, so we have limited time. With my age-group I would prefer to have an hour. But if we ran it after school we wouldn't get much attendance.
We also have links with reading groups in other schools and book awards run by the county library service.
If you have any specific questions, do fire away and I'll do my best to answer them.
If you want ideas as to how to get children talking about books this book is a good place to start.
I have various resources around in my office at work, some of which are great, some of which at least get you thinking of other ideas. If you want me to, I'd be more than happy to put together a pack of materials to set you off with a few ideas.
If you're interested in this, just CAT me and I'll sort it out.