select offset by alphabetical reference

Have a table with Last, First, etc...
wish to create a select to grab everything from table whose last name is
alphabetically greater than 'Smith'

almost like the following (which is obviously incorrect);
    select last,first from mytable order by last offset 'Smith';

ideas on how to handle this at the postgres level rather than grabbing all and
storing it in an array in PHP/Perl for post processing to grab the desired
records?

thanks

Dave