Oliver,
Create a folder under c:\wamp\www
For example c:\wamp\www\firstsite
Put your hello.php file in that folder ( this keeps your work seperate to the wamp code)
You can create as many folders under there as you like for different sites.
Then use this address in your browser
http://localhost/firstsite/hello.php
Even better change your filename from hello.php to index.php ( that is the default filename to run in a webserver "normally" )
then you can just use this address
http://localhost/firstsite
BIG NOTE::: DONT PUT index.php in the c:\wamp\www folder wamp has already got on in there tha you should NOT OVERWRITE!!!!