How do I compact my messages as I have ran out of space

If this is an IMAP account, you could do this:

1. click help>troubleshootinginformation
2. scroll to 'profile folder' and click 'open folder'
3. exit thunderbird
4. click imapmail\<IMAP account folder>
5. locate the large message folder, probably inbox
6. DELETE the inbox and inbox.msf entries
7. restart thunderbird

If a pop3 account, ensure the trash is empty, try and delete a few message and empty trash ie. clear a bit of space.

The compact a small folder to make more space etc.

GUI is File > Compact Folder, just don't start with the overall Inbox as it needs pace to start

If the space that you have run out of is in your account on the server, compacting will probably not help. You will have to delete messages, after copying them to local folders in Thunderbird, if you want to preserve them.