Are you intending to become a forensic psychologist? Or do you have a different career path in mind?
Where do you work at the moment?
Volunteering of any kind will be helpful, the uni's will like it if you're showing general dedication outside of school and especially if it is 'psychological' (in the sense of helping others). Older people's homes, Barnardo's and other charities, childline, befriending services - there's quite a lot out there.
If you want something more forensic orientated, volunteer with the Youth Offending Service, some great opportunities both in terms of working with people and becoming familiar with how the law is actually applied.