Hi Rithesh,
First you try to make a prototype and then think about how to install it in a real door or lock.
Take a wire out from the circuit where the touch pad is connected and then connect the wire to the metal lock.
Try to finish the touch circuit first and then start working on the Arduino. With in 2, 3 days we are uploading arduino projects based on SIM900 and you can find examples of the same under
EG LABS> MICROCONTROLLER > ARDUINO.
Suggest you to start with the touch switch circuit and proceed step by step