Part of engineering is to break up projects/problems in to smaller parts, and work on the smaller parts.

With the GSM/3G phone or module.... why don't you connect it to the serial port of a pc and send command by Hyperterminal 1st. Do a google search on the GSM AT command set... or read the manual of the GSM/3G part you are using.

Then when you are confident that the serial commands and the GSM/3G phone or module is working on the network you can then progress with connection to an 80c51 and then 80c51 serial commands to send SMS's.

Further help will require exactly which GSM/3G phone or module you are using and which 80c51 micro you are using.

Joe