Your referee will typically be:
A family member or someone who is only a friend cannot act as your referee.
If you are currently studying, or have left only recently, your referee should ideally be a tutor from your school or college who can comment on your ability to succeed on the course. Universities and colleges require an academic reference and it will be difficult to gain a place without one. If you have recently left, it might be worth speaking to your school or college to discuss making your application through them.
If you are not currently studying, your referee should be someone who is able to talk professionally about your application and potential, such as an employer or training officer. Your reference does not have to come from an academic source.