how old are you? where do you live? what are your skills and talents you have to offer? can you drive? what needs have you seen in your previous work that seem to be unmet in your area? For instance one of our students volunteered in a hospital for her Confirmation service and realized there was a need to instruct patients going home, and their caregivers, in Spanish about medications, home care etc. so she asked for that job and now there are regular volunteers trained who do that for every non-English speaker (she has gone on to the Peace Corps in a public health project in Africa).
As for making a difference, don’t worry about making a global difference, think about making a difference for one person at a time.