pls check the following link interface sim300 gprs modem with 8051 microcontroller
Embed4u » Code Library
#include<string.h>
#include<reg51.h>
unsigned char x=0,i,g[20];
unsigned char s1[9]="AT\r\n"; //send "AT" command modem will response
unsigned char s2[18]="AT+CIPMODE=0\r\n";//Select TCPIP Application mode
unsigned char s3[30]="AT+CLPORT=\"UDP\",\"3500\"\r\n"; //set Local Port
unsigned char s4[45]="AT+CIPCSGP=1,\"airtelgprs.com\"\r\n";//set csd or GPRS for Connection mode "ur GPRS provide address"
unsigned char s5[15]="AT+CSTT\r";//Start Task and set apn
unsigned char s6[15]="AT+CIICR\r";//Bring up wireless connection with GPRS or CSD
unsigned char s7[15]="AT+CIFSR\r"; // Get Local Ip Address "is assign for GPRS Modem"
code unsigned char s8[50]="AT+CIPSTART=\"UDP\",\"219.64.118.125\",\"3500\"\r\n"; // Start Up TCP or Udp Connection
unsigned char s9[15]="AT+CIPSEND\r\n"; //Send Datat Through TCP or UDP Connection
unsigned char sd[46]="193308950\r\n"; // This is the Example Data Send through GPRS Modem