Getting started:
shell> mysql --user=root mysql
#   using 'GRANT' adds a user
mysql> GRANT ALL PRIVILEGES ON *.* TO monty@localhost
    ->     IDENTIFIED BY 'some_pass' WITH GRANT OPTION;
mysql> GRANT ALL PRIVILEGES ON *.* TO monty@'%'
    ->     IDENTIFIED BY 'some_pass' WITH GRANT OPTION;

mysql> GRANT USAGE ON *.* TO 'beancounter'@'localhost' IDENTIFIED BY
	'Acme543' ;
mysql> GRANT USAGE ON *.* TO 'beancounter'@'%' IDENTIFIED BY 'Acme543' ;

mysql> FLUSH PRIVILEGES;
     -or-

shell> mysql -u root mysql
mysql> SET PASSWORD FOR root@localhost=PASSWORD('new_password');
mysql> FLUSH PRIVILEGES;
     -or-

shell> mysql -u root mysql
mysql> UPDATE user SET Password=PASSWORD('new_password')
    ->     WHERE user='root';
mysql> FLUSH PRIVILEGES;
     -or-

shell> mysqladmin -u root password new_password

Lock a user out right now:
REVOKE ALL PRIVILEGES,GRANT FROM user_name [, user_name] ;
FLUSH PRIVILEGES;