Look for a directory "htdocs" in your installation (somewhere under the Xampp installation folder) and put a simple test file into that folder, lets call it test.php and insert any trivial coding (mostly 'echo "Hello World";' or similar).
Start Xampp (at least Apache).
Start your browser (Safari?) and in order to view your first test file, enter http://localhost/test.php
in your browser (you may also simply click on the link). That is the first most important thing, put any files into the htdocs directory and you may see the working result via http://localhost/filename.xyz
You may also create any subdirectories in any level under the htdocs folder and put your files there, this results in http://localhost/mysubfolder/filename.xyz
(where mysubfolder of course is only a placeholder for your own subdirectory created before).
Of course there are plenty of options and configurations to change standard Apache behaviour on your PC, but this will take a long time to learn and to use them and fortunately, for your first own files (or even for your whole live) you dont need to change the configuration. There is only a possible problem with the accessibility of the created files, as Apache runs under a certain UserId, the folder htdocs has certain rights and you are running you own UserID. You may change any of the file and directory rights, you even may change ownership, if there are not sufficient rights for Apache, it cannot open the file and this will lead to a runtime Error (i.e. Error 403). This sometimes is a hassle for OSX Users, as they are not used to work an system level. But If you are struggling, you can ask here again.