Mine was probably the 'dubious' credit of the Snails at Joyland Gt. Yarmouth, some say it is, some say it isn't. Myself on said ride many years later
Obviously RCDb disagrees with this, however, which would mean that my first coaster was the Big Apple at Alton Towers (now operating, funnily enough,at Pleasure Beach Great Yarmouth). My first 'proper' coaster was Klondike Gold Mine at Drayton Manor, though I probably didn't become a fully fledged enthusiast until experiencing the (then called) Dueling Dragon coasters at Universal Islands of Adventure is 2005. I've never looked back since.