first it would be interesting what you already know? Do you know how to write HTML? And do you know any type of Programming Language?
For the very first beginning go to the directory where you installed xampp. If you installed it to C:\xampp, then go into this directory:
there you create a folder (name doesn't matter), for example "test". Be careful with upper- and lowercase, it is very important! Inside this folder you create a textfile and name it to index.html. Be sure that it is an html file and not hiding it's extension. To ensure this open a Explorer Window (NOT Internet Explorer) and in the Menu go to Tools -> Folder Options, in the Tab View you search in the List for the Point "Hide Extensions for known file types" and uncheck this, if checked. If after that your file is named like index.html.txt, then rename it to index.html.
Now open that file in a Text Editor (for example Notepad) and type whatever you want.
Now open your Browser and type http://localhost/test/
as the address. The content of your index file will be displayed.