what do I need to delete to purge the backups 

First aproach will be to reduce the retention on your policies or plans.

You can also delete the jobs from each copy of your storage policies or plans.

More drastical will be to delete directly the storage polcies or plans defined.

And run the data aging administrative job in all cases.

@Robert Dorans If you are using disk library, then you can view the disk library summary to identify what is occupying more space.

https://documentation.commvault.com/2023e/expert/viewing_disk_library_summary.html

You can also check the Data Aging and Forecast report to identify which jobs are about the age.

https://documentation.commvault.com/2023e/expert/data_retention_forecast_and_compliance_report_overview.html

Based on this information you can decide which jobs can be pruned.