The MOST very simple: On your computer open a windows console (or linux console, whatever OS you have) and type "php abc.php" where "abc.php" is your file. For this to run, you have to have PHP path in your "path" variable, otherwise you will get something like "no such program 'php'". If you're on windows, if your PHP is installed for example in "c:\php" then you need to type "c:\php\php.exe abc.php". If you're on linux, this won't be needed. It will be within the path I think.
Very simple: Just type the path to the file from the perspective of the web-server into the address bar of your browser.
Example: You got a web-hosting on "www.example.com". And you created "abc.php". And you copied this file into the main directory of your web-server site directory. Then you simply type "www.example.com/abc.php" in your web browser. If you rename your file to "index.php" you don't need to type its name, just the adress "www.example.com" as "index.whatever" is special name for the webservers, just like "main.whatever".
I guess your web host runs on Apache server, so you need to copy the php file to your "wwwroot" directory on the web-server.
Also important - the php file need to have "execute" and "read" attributes set for the user group of the web-server.
That's it. If you can't do it - it's time to read the books and the help files.
Good luck!
PS: Of course this is assuming your PHP program is working fine. Because if it's not and you run it in the web-server you will get HTTP Error 500 (which is "Internal Server Error") or a full description of the error, depending of how the web-server is configured to answer. If you get something like "Access Denied", then check your web-server directory permissions and the permissions of the file too.