Computers send information through the Internet by dividing the data to send into small chunks ("packets") and transmitting them to the other device. All this happens without your doing anything - the web browser, e-mail program, etc. all take care of these low level details.
When your computer wants to send to another computer, it creates the packet, places the other computer's address in the destination address of the packet, places its own address in the source address of the packet, and sends the packet, either directly to the destination computer, or to a nearby router that takes responsibility for routing the packet.
To use a post office analogy:
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