I think the biggest thing is to just ask people you know and reach out to organisations that you are interested in. All of my volunteering opportunities have happened by chance.
1) Respite Care
An older friend of mine who was involved in a respite grouo found out I knew sign language and suggested I come along. I love the children and now you cannot tear me away. The time commitment is only three hours a month, but it makes such a huge difference.
2) Pre-Prep
Pre-Prep is a program for school-aged children who aren't quite ready for school due to mild intellectual disabilities and developmental delays. Our school had each year ten do a day there, and the coordinator of the program saw how much I loved it and said I would be welcome back any time. Next term I plan to pop in once a week for forty minutes or so on my way home from school.
3) YouthFest
YouthFest is a festival for young people in my area. They take on heaps of volunteers and are super helpful in making it easy for you to stick it on your resume. It's just a really great day and the options for what you do are super diverse. From photographer, to social media stuff, to environmental patrol to inclusion stuff. There is one prep session which takes an hour or so and then it's a three or four hour commitment on the day.
4) Cooking Classes
Just by chance, I was chatting to a family friend and she told me that she was involved in running cooking classes for adults with disabilities. I told her about my work and she said she was always more than happy to have me. I've been three times and every time has been better than the last. It's three hours whenever I choose to go, with no obligations.
5) School Things
There are so many opportunities at school to volunteer your time by tutoring someone else. Maybe you have a friend who is struggling or know that a friend's little sibling needs help with a subject you ace. While I don't know how much it would boost your resume, it is certainly a way to consolidate your knowledge and give back positively to your community.