Welcome to Android Central! To select a new ringtone, go to
Settings>Sound>Phone Ringtone, and select a new one.
To use your own MP3, transfer the MP3 file to the Ringtones folder. You can connect your phone to your computer via USB and use your computer to drag and drop the file to the Ringtones folder. If the MP3 file is already on your phone, you can use a file explorer app like
ES File Explorer to move the file to the Ringtones folder. When you go back to the Ringtone settings menu, you should see that MP3 available as a ringtone. If you don't, try rebooting first, then check again.