If storage usage of org is getting full then how to free space in salesforce ofg?

Hi,

This simply says the data storage allocated for your organization has been exceeded. You have to delete records and free up some space or purchase additional space. You can see your available data storage by navigating into Setup > System Overview. And you can monitor your usage by navigating into Setup > Administration Setup > Data Management > Storage Usage

If you can confirm some records to be deleted, Mass delete record feature will helpful for you. You can find it at Data Management > Mass Delete Records under Administration Setup (You should have required permission). There maybe lots of Task and Event records hopefully where you no need. You can delete them and free up some storage.