If your GSM module has Rx and Tx pins (TTL) then Cross it to MCU Rx and Tx pins. If Your GSM board has RS232 connector then you need to have a MAX232 or MAX233 (better) between your MCU and GSM's RS232 connector. You need to write Serial receive and send code and use Serial send code to send the AT Commands to the GSM module. Google for "8051 GSM projects".
Post your GSM modules datasheet, manual, AT Command manual, etc...