The command line option from SSH is:
mysqldump -d -u -p database_name > file.sql
Where "database_name" is the name of your database, and "file.sql" is the name of the file you would like to write the table structure to. It will prompt you for a password (-p) and this is required. Normally mysqldump gives you the entire database, with the "-d" option, you tell it to not include the data, but just the table structure.