well, assuming you're logged into the computer you should be able to just execute the mysql binary (either from bash or dos) and it might let you in without any password (depending on wh o you log in as...)
mysql
(or ./mysql)
once you have logged in execute the grant command to specify a new user and the permissions that user has on the database... If you can't log in then get whoever knows the root password to log in and grant you permissions, then log in as that user to mysql next time.
by default there is no default password on a linux /gnu box, although if the sysadmin hates you try "admin" or "password" 🙂