There are many different ways to volunteer from helping out at your child's school through getting involved with your local hospital's radio station to doing VSO (Voluntary Service Overseas) in your gap year. Whatever your interests, and however little time you can spare, there is a volunteering opportunity near you.
Why volunteer? Well I know from experience that not only is volunteering immensely satisfying in itself, but you can make a real difference to people. Bring some friendship or comfort to someone lonely, help improve the environment, teach children in a third world country (or just here at home). Almost whatever it is there is an opportunity for you in your local community or much further afield.
And as someone who is involved in running two, totally different, voluntary organisation I know that both small local organisations and national charities are always in need for more people to help. And I also know that volunteers really do make a difference.
There's a lot, lot more information about volunteering, and Volunteers' Week, over on the Volunteering England website. Find out what there is near you!