You can create ringtones for your Windows Phone 8 but if you have one of the newer Lumia phones from Nokia, like the Lumia 920, Lumia 820 (and its Lumia 810, Lumia 822 variants) you might be interested in checking out this Ringtone Maker application from the company that branded your phone.
It is available in your Nokia Collection (as well as accessible via the link below) and, as the description states, it will help you create ringtones. You can do so by using music on your phone which can be saved as a ringtone by opening the app and selecting the MP3 file. Of course you can trim the audio to your hear’s content but according to reports you can not use an Xbox Music file (downloaded or purchased).
Source: Windows Phone Store