now my question is: how do i start that program? do i have to give some command in the first line of the program, or do i have to use a starting address, or will it start by itself, or do i have to make a pin on the processor high or low?
1) with a question like that you MUST read the '51 "bible" the links to chapters 1-3 are in links <----. No need to download the whole thing, it is humongeous.
As for your question.
on power up and the reset from the supervisor chip active, then inactive the '51 will execute the instruction at address 0. This is the kind of information that may get you in trouble (overwriting interrupt vectors etc.) Thus I urge you to get the whole story from the "bible"

Have fun,

Erik