With WordPress, you normally log in at yourname.com/wp-admin/.
If you do not know your password, you may use WordPress’s password reset tool. Go to the log in screen (the URL above) then click the link for resetting your lost password. You’ll receive an email with a link to reset your password.
If you don’t have control of the email address on the account then you can usually use your web host’s tool for managing the WordPress site’s database to reset the password. This tool is usually called phpMyAdmin.
See Resetting Your Password at WordPress.org for more on these and additional methods. Also know that sometimes a web host will have a tool in your account for easily resetting your WordPress password.