OK. What you need to do is use your file manager app, and see if you can navigate to a folder called Ringtones. If not, create a new folder within the file manager app.
Now, you need to download a music file and save it in that Ringtones folder. Since it's going to be a ringtone, use your contacts app and navigate to the contact you wish to set this ringtone, and edit the field for ringtone to your downloaded file. Save and now when you receive a call by them, your downloaded ringtone will play.