well first you need to write a test script.
test.php
<?php
phpinfo();
?>
then, you need to place this script in your htdocs folder. hard to say where it might be, depending on your distro it could be at /usr/local/apache/htdocs.
once you have this script in place, you'll need to open your browser, and in the address bar type http://localhost/test.php. if you get a bunch of info about php, then your up and running.