every town should have a "CVS" this is a council for voluntary service. if you google "your town cvs" or "liverpool cvs" for eg you will get contact details.
the cvs will know of most of the voluntary, charitable organisations in your area - and they will ask you what you are interested in helping out with - so a youth club is much different to helping the elderly use computers - they will sort you out. they will also know of the best resources. so if you did want to volunteer with young or vulnerable people they would be able to give you advice on how to go about being policechecked. they will kow of upcoming training which may benefit you and the organisation you are helping