I have 20gb room at my server. I have used 19.988888888gb. I can not do anything without errors. I have to delete something else or pay for more.
In Administration > Site configuration > Site logs you can display all (no filters) then click “Delete”. This should empty that table and probably save a large amount. If you are unable to do this in admin, you can do it in phpmyadmin. Just empty the whole table. It will not matter much to lose these entries.
Also in Administration > Site access you can click on “Delete” in the bottom half. That might also give you a lot more space. DO NOT try to do this one in phpmyadmin – you risk locking your site completely of you delete the wrong records!
Then check you media folder. Perhaps you have large images than can be made smaller without loss of detail?