You may refer this useful example.

http://opensees.berkeley.edu/wiki/index.php/Laterally-Loaded_Pile_Foundation

For the "interface area" the easiest way is to click on "Find Contact Pairs" button in Interaction module panel (flashlight icon) and it will find your contact surfaces automatically, within the separation range that you are defining.

For "interaction", in addition to interface area, you need to define interaction property, in which, you define mechanical properties of contact (e.g. friction, cohesion,...) and assign it to surface pairs. you cn define your interaction property in the interaction module and "Create interaction property" button

You may refer this useful example.

http://opensees.berkeley.edu/wiki/index.php/Laterally-Loaded_Pile_Foundation