Locate the Tools tile and select Licenses In-Use / Storage. If you do not see this tile, you may not have permission to view it. Contact your office administrator to give you permission.
Within the Licenses In-Use / Storage, below the licenses and the users logged in you will find your storage information. Hovering over the graph will show percentages of Used vs Available storage.
This is only a calculation. If your company is over, you will still be able to upload attachments.