1) start MAMP (Applications->MAMP->MAMP)
2) read the readme.rtf
3)
1. make a file called 'info.php' in you favorite editor.
2. put the following code in this file:
<?php
phpinfo();
?>
3. put the info.php file into the folder 'htdocs' in de MAMP folder
4) access it via 'localhost:8888/info.php' (no http in the browser bar is needed)
now you should see a page with a bunch of information. Now what version of PHP have you installed?