The users manual and help functionality in the MyCentro app on the phone are really quite useful and explain pretty well how to get ringtones onto the phone (many different methods). Many more options if you have a MicroSD card installed.

Just realized you said you had songs on the phone. Are you wanting them as ringtones? If they are MP3's on your MicroSD card you can use an app called Minitones to edit them and use them as ringtones.