MP3 files and ringtones have different file formats. A conversion is required.
If you have a Mac, GarageBand can do the conversion. If you have a Windows PC, there are third party products to do the conversion, but I cannot personally name one.
You will need to convert the mp3 file into a file that is usable on the iPhone as a ringtone. There are several steps involved, and you would be better to resort to Google to look these up. You will need to convert the file to an AAC, then change the file extension, get it into iTunes and then sync to the phone. Those are the basic steps, but you would be better to search for the more detailed version.
As has been said you will need to convert the MP3s to AAC format. However iTunes itself can do this just fine.
Chris CA details the steps in this thread.
Quite fiddly and technical i must say. Why has Apple decided to implement this system?
With Android (and this is for all of their phones) i just tap on the songs and an option appears to "Set as Ringtone" and that's it.
As someone who came from Android and now loving his iPhone i wish Apple has implemented a less-technical (or long) way to do this thing.