Consider the focus of your book club. Do you want to explore classics? Nonfiction? Memoir? Mysteries? Sci Fi? A certain author? Food? I started a book club just for parenting books -- mainly becuase there are so many great parenting books out there, but what parents has time to read them all? We got together and those who hadn't had time to read the book got a lot out of it just through our discussion.
Having a book club that is open to reading any kind of book is also a focus. Just be sure everyone is on the same page about what they want.
Decide how big you want your book club to be. I have found that in smaller book clubs (6 members max), each person -- even the shy or more reserved ones -- gets a chance to speak and everyone gets to benefit from everyone's wisdom. Larger book clubs tend to become more raucous, and can degenerate into get-togethers where one or two more outgoing people do most of the discussing and everyone else is just observers.
Consider when you will have your book club and where. First-Tuesday-of-the-month kind of thing is easy for members to remember and plan around. (Send out an email the day before anyway.) Some book clubs rotate, giving everyone a chance to host, but that means a lot of energy is spent with everyone trying to figure out where it is each time. Having the book club meet at one location each time makes it much easier for members to make it after a hard day of work, but is a big commitment for the one host.
Consider how much you actually want to focus on books. Some book clubs are heavy-duty literary discussions; others are dinner parties or wine and cheese affairs where the book may come up in conversation -- or may not. Decide which suits you best.