Claire, not sure which end of Brentwood you are, I used to live behind the High Street, but pop into Yellow Jersey 25 High Street BILLERICAY, they do social rides on both Saturday and Sunday morning
Hello Claire and welcome. You dont need to join a club, as Groucho Marx famously said 'I dont want to belong to any club that would have me as a member!'.If you haven't cycled much before just start with some local routes of five or six miles and gradually build up. Take it at your own pace, it'll be tough at first but you'll soon be doing distances you couldn't previously have imagined. Maybe you could find a like minded person locally and team up for rides.
If you do fancy a club ride I see Essexroads have an event on 16 July starting in Billericay:
http://www.essexroads.com/events/sunday-club-ride-2017-07-16/
There's also a club in Brentwood.
Good luck, any other questions please ask.