I presume "sim908" is some kind of cell-phone shield?
From my experience you usually send those things a bunch of AT commands via serial communication.
For starters you can ignore what the board sends you and just issue the AT command to send a text message (SMS) with the string you want to appear on your phone.
If you're using a single-serial port board such as Arduino UNO, keep in mind that the USB cable takes up the only hardware serial port, so if you want to keep using e.g. the Serial Monitor you need to use a software serial port to talk to the cell-phone shield.
"send those things a bunch of AT commands via serial communication"
i meant something like Serial.print("AT+ whatever")
with the only (key) difference that if you need to use software serial you can't actually write Serial.print(), otherwise the strings will appear in Serial Monitor instead of being sent to the GSM shield.