How does one set up an SMS-to-computer-program interface, so that I can handle any incoming SMS messages to a particular number via a computer program of my choosing?

In other words, I want this to be able to happen:

* Somebody sends an SMS to 1234567890 (or shortcode 1234).
* My program receives the SMS and has full access to the sender's phone number and content of the message.

Thanks in advance for *any* help.