If you are sending it to a different country, you have to fill out a form. You can either wait until you are ready to send it, or you can fill out the form before going to the counter. They will have blank forms available next to the counter for anyone to take. On the form, you have to specify what you are sending, how much it costs, and the address of where you are sending it to. After filling out the form and packing your box, you are ready to go back to the post office to send it.
Because the box should already have the address, they will not ask you where you are sending it. The first question they usually ask is how you want to send it. You can send it by boat, and a variety of air mail.