There should be a link on the login page for forgotten password. If you provided an email address when you first setup WP, then the program will email you a new password.
Alternatively, access the MySql database, and Browse the wp-users table. Older versions of WP will have your password in clear text. Newer versions show the password as a long string of letters and numbers, which is entirely useless to you. What you can do is edit the record and delete the password. That should allow you to login with just a username. Immediately after you login, edit your user profile and assign yourself a new password. I haven’t test this, but it should work.