org 00h is a directive that tells the assembler where to write the program into the micrcocontroller's program memory. By using a data address of 00h, org tells MPLAB to store the first program instruction into the first memory location (address 0000h), which is also known as the Reset Vector.
The Reset Vector in the PIC is the first memory location that the microcontroller goes to on power-up or after a hardware reset, so the first program instruction must always be stored there. In C lang using C18,i think is start from 00h only. No extra instuction is required.