Depends on what area you are thinking to volunteer
I work through the YOT (youth offenders team) with the young offenders
As an Appropriate Adult within the Police Custody Suite
As a Befriender where you give some of your time every week/every other week depends on the Social Worker - and you are allocated a young person - you then take this young person away from their home environment and do "normal" things - This is rewarding because you see the child in them and happiness - even if its only for a few minutes/hours.
You need to initially decide what area (children/adults etc) and also if you will be needing a CRB check - get that sorted they take ages!!!! - you will have to pay unless the place you volunteer offer to cover the cost!
GOOD LUCK and hope you find something rewarding