How do I become a friend?

Becoming a friend is the same as adding a friend.

Facebook answers the question of how to add friends in the Adding friends section of the Causes Help Center.   Their answer:

To add a friend, just use Facebook Search to find people you know and then click on the "Add as Friend" button to the right of their search listing. A friend request will be sent to that person. Once they confirm that they actually are friends with you, they will show up on your Friends List.