Presumably you have Apache installed and that's what you use as your main web server (ie, it listens on port 80). If you have not done (much) customisation to your Apache install, it should be accessible at:
(If you're accessing the server from a different machine, then obviously substitute
The installer for phpMyAdmin installs a config file into
phpmyadmin, which sets up an alias. Look at this file to see how it works and where it should appear on your website.
If you have modified your Apache configuration, it's possible though unlikely that you have done something which overrides the directives in that phpmyadmin config file. In which case you should be able to fix phpmyadmin's configuration yourself to get it working how you like it.