I have mounted on the GSM add-on v2.0(sim 800) on the pi. I have read the AT commands manual but don't know how to initialize the serial connection between both of them till the point AT commands have to be entered. I can't find any information online. Any information will be greatly appreciated.
If you use a recent version of
raspi-config, you will find there is an option to prevent taking over the serial port as a console.
Then it is free for you to use as a regular serial port