Someone else's boat

There are many charter opportunities, sailing schools and youth training schemes
Probably every sailing club in the world has options to sail without your own boat, whether hire or crewing for others.
Many people, whether or not part of a club welcome volunteer crew ... but there's work to do.
The RYA's "Push The Boat Out" scheme demands special mention.

Sailing clubs

 

There are sailing clubs all over the world and many (most?) cater for we mere mortals with modest pockets.

In the UK, most or all such clubs will be affiliated the the Royal Yachting Association (RYA), which provides advice, support and information for boaters of all kinds from the smallest dinghies upwards.

They are the main authority in the UK that set down the syllabus and expectations for a number of boating-related educational courses. These cover most things that my be of interest to boaters from their introductory "push the boat out" days, though courses for windsurfing, dinghy sailing, cruising, power boating, navigation, radio operation and much more.  Explore their courses: https://www.rya.org.uk/training

They have a register of clubs and and training centres here: https://www.rya.org.uk/wheres-my-nearest