Event ID: 1105 MSSQL$SQLEXPRESS - primary filegroup is full.

Dear Fellows, 

Event viewer full of the error event id 1105, for sharepoint mssql$sqlexpress. 

"Could not allocate space for object 'dbo.AuditData'.'AuditData_OnSiteOccurredEvent' in database 'WSS_Content_91f6fce9594a4facbebac2e8bc29b90f' because the 'PRIMARY' filegroup is full. Create disk space by deleting unneeded files, dropping objects in the filegroup, adding additional files to the filegroup, or setting autogrowth on for existing files in the filegroup.".

Please advise me the suitable step by step resolution.

Thanks,