How to add my own music as ringtone on iphone 5?

As above i can't seem to work it out. I'm new to all things apple and am getting really confused.
many thanks.

Added (1). Jan i did as you suggested and downloaded a ringtone app & its worked.

Go on AppStore and search for '' ringtone'' or something like that and you will see. Usually its like you put your song on the app and then they send the ringtone version to your email so you download it into your laptop and then sync it on to iphone.