Welcome aboard, thingamabobs!
Cruises are amazing and awesome and lots of fun and yes, you'll be planning your next one while on board. (Do it early before the lines get too long. It's cheaper if you book your next cruise while on board.)
Basically, the way Fish Extender groups work is that once there's enough interest here (probably in this thread), someone will set up the FE exchange list and people can sign up for it. You hang a so-called "fish extender" on the fish (or seahorse) outside your stateroom and other members of your FE group will put stuff in it, and you'll put stuff in theirs. There's a lot of flexibility in terms of how it's done, but generally it's expected that you'll give at least one gift per stateroom for the cruise. Some people will do one gift per person per cruise, others do a gift per day - it's entirely up to you! Most people will sign their names and stateroom numbers as well.
Group size depends on the number of sign-ups, but most groups I've seen are 11-16 staterooms (so you're giving 10-15 other people gifts.)
There's also the practice of "pixie dusting", in which you put small random gifts in random FEs not necessarily in your group, with a little note saying "you've been pixie dusted!" or similar. This is completely optional, but pretty cool.
