Mentors are the people who will be recording messages of support for you, so you might choose someone who you look up to, and/or are inspired by. You should trust the mentors you choose as well. They can be a parent, grandparent, friend, sibling, child, co-worker, etc.
At the very least, you can add yourself as a mentor to record your own messages, as well as ask your Sixth Dimension Coach (the person who introduced you to Sixth Dimension and who you signed up under). He or she would be more than excited to be one of your phone mentors.