Think about which population (children, elderly, homeless, mentally challenged, etc.) you'd like to help the most, or help the population that you plan to work with as a social worker.
Gather the names and contact information of at least three personal or employment references. You might or might not need them. Update your resume in case someone asks for it.
Google your city's name with "volunteer center" to see whether it has a volunteer center -- for example, search "Dallas volunteer center." If you find one, contact them and they will assist you Contact your local United Way and ask which agencies currently need volunteers the most. Visit volunteermatch.org and search for opportunities. Keep in mind that not all agencies and jobs are listed.
Consider becoming a CASA (Court Appointed Special Advocate), which involves visiting a child in foster care, attending monitored family visits, gathering info, going to CPS meetings and (in some cases) testifying at court hearings. This is a great way to get experience if you would like to become a CPS worker.
After getting a volunteer job, treat it as you would treat a real job. Although you might not be getting any financial compensation from it, the resume experience and joy that you will bring to someone else are priceless.