PIC is where you are the pilot in command. Any training flights with an instructor on board will be logged as Pu/t (pilot under training). Any training flights where you are the sole occupant of the flight are logged as P1 or PIC. Obvious as you are the only person there, ergo, you are pilot in command. Hence, your first flight as PIC will be your first solo. Every subsequent solo flight will be logged as PIC. Don't worry too much about this as the moment as your instructor will explain how to correctly log each flight.
Good luck