After booting Linux, you should be presented with a login prompt, like so:
At this point, either the distribution's documentation or the system itself will tell you what to do. For many distributions, you simply login as root, with no password. Other possible usernames to try are guest or test.
Most newly-installed Linux systems should not require a password for the initial login. However, if you are asked to enter a password, there may be a problem. First, try using a password equivalent to the username; that is, if you are logging in as root, use ``root'' as the password.
If you simply can't login, there may be a problem. First, consult your distribution's documentation; the username and password to use may be buried in there somewhere. The username and password may have been given to you during the installation procedure, or they may be printed on the login banner.
One cause of this may be a problem with installing the Linux login and initialization files. If this is the case, you may need to reinstall (at least parts of) the Linux software, or boot your installation media and attempt to fix the problem by hand---see Chapter 4 for hints.