Deciding what services/products you offer is the easy part. Getting the paper work together and a chunk of change to start it, that's the real toughie.

Let me recommend the NCBA as a great resource for how to start-up:
http://www.ncba.coop
Their startup list is awesome. It's here:
http://www.ncba.coop/abcoop_howto.cfm

the sad thing is, co-ops need seed money, need to think about things like insurance coverage, need to write bylaws, need a way to secure a space, etc. lots of logistics to wrestle!