A user can deposit a text message together with a destination number on a web server. Your arduino then periodically gets waiting messages from the web server and sends these to the destination number via its GSM shield. Is that right ?
If that is correct, you have to use the Arduino to contact the web server using the GSM shield (in web client mode) and say the HTTP get method to obtain the next message to be sent, then again using the GSM shield, dispatch the message. Optionally, send a confirmatory message back to the web server that the message has been successfully sent.