which are two basic point of reference on the surface of the earth

That depends on what reference level you have chosen. If the (arbitrary) reference level you chose is Earth's surface, then anything on the Earth's surface has zero potential energy. If you choose some higher reference level, an object on Earth's surface has a negative potential energy. If you choose a lower reference level, an object on Earth's surface would have a positive potential energy.

reference point

The reference line is the normal (perpendicular) to the surface.

The reference line is the normal (perpendicular) to the surface.