Hi, This is simple and also depends on your Editor. This is what you do. In the same way you created your "index.html" file then all you do i make a css file by clicking on "save As" and then in the box that names the file before you finish saving your file you just type this in.
style.css then it saves as a .CSS file ok.
in the same way you created your "index.html" you creat your "style.css" its all down to the last letters after the .DOT ok that makes a file what it is.
Hope this helps!!