The vanilla installation has no root password, so you can do:
# mysql -u root
However, really securing the installation is more involved than simply setting a root password. To make sure "all the bases are covered" (to use a U.S. idiom), use this command:
This is an easy way to effect basic setup so, for example, root can login only from localhost, etc.
If you're not particularly concerned about security, you can put the root password in
/etc/my.cnf-- that way, you won't have to type it manually.