A few ideas for ya:

1) Collect newspapers and donate them to a nearby shelter (for lining cages).

2) Collect used collars, leashes, dishes, toys, and beds from your neighbors and friends that have pets.

3) Save a little bit of your allowance, baby sitting/ pet sitting money, chore money, etc. and put it towards a charity fund. A dollar a week may not sound like a lot, but every little bit helps.

4) Convince your parents to buy a bag or two of pet food to donate, every once in a while.

What is the age requirement for volunteering? I'm sure there is something out there for you. Ask around at vet's offices, animal fostering organizations, and shelters. I hope you find something.

Good luck to you.