Solution
I am just posting the solution if anyone needs it. Everyone seems to downvote the problem instead of giving solutions.
- First of all, remove all the jumpers [see the picture]

Then add two Male to Female cable to the S-TX and S-RX [Here S stands for SIM900, you can find the pins after removing the jumpers.]
Short the grounds of both arduino and ATK-SIM900A module
S-TX must be connected to the arduino RX and S-RX must be connected to the arduino TX.
Normally power up the module [Don't forget to press PWR Key
afterwards]
Code
#include <SoftwareSerial.h>
#define BAUD 9600
#define RX 10
#define TX 11
SoftwareSerial gsm(RX, TX);
int8_t sendATcommand(char* ATcommand, char* expected_answer, unsigned int timeout) {
uint8_t x = 0, answer = 0;
char response[100];
unsigned long previous;
memset(response, '\0', 100); // Initialize the string
delay(100);
while ( gsm.available() > 0) gsm.read(); // Clean the input buffer
gsm.println(ATcommand); // Send the AT command
x = 0;
previous = millis();
// this loop waits for the answer
do {
// if there are data in the UART input buffer, reads it and checks for the asnwer
if (gsm.available() != 0) {
response[x] = gsm.read();
x++;
Serial.println(response);
// check if the desired answer is in the response of the module
if (strstr(response, expected_answer) != NULL)
{
answer = 1;
}
}
// Waits for the asnwer with time out
} while ((answer == 0) && ((millis() - previous) < timeout));
return answer;
}
void setup() {
// put your setup code here, to run once:
Serial.begin(BAUD);
gsm.begin(BAUD);
}
//Code for sending sms, replace the stars and country code with your corresponding phone number
void loop()
{
sendATcommand("AT+CMGF=1\r\n", "OK", 500);
sendATcommand("AT+CMGS=\"+880******582\"\r", "OK", 500);
delay(500);
gsm.println("Hello world"); //The message to be sent
delay(500);
gsm.println((char)26); // Done
delay(500);
}
Thanks to all.