I am an absolute beginner and wanted to build a simple home automation system using DTMF module using this guide. My problem is out of 12 pins most of them are getting used in DTMF module(5) and the relay module(4). So how do i add an LCD display to it?
You are using 8 GPIO pins for your existing modules. That leaves 12 left for an LCD (assuming you aren't using the serial port).
An LCD can work from as few as 6 pins (4 data, E and RS) - WR can be tied LOW and RESET can be tied HIGH, and the remaining 4 data pins left floating (or tied low).
That still leaves you 6 pins to do other things. Or 4 if you (advisedly) steer clear of using pins 0/1 for anything.
I suggest you first try every component separately (or if you are new to Arduino, try first simpler components like LEDs, switches etc),
As told by others the following components are easier to use:
Use a matrix keypad, see for example keypad. To reduce the number of pins you can use a multiplexer, such as 74HC165. 74HC165 are very cheap, although they need some additional programming and are slower than using direct pins (although for a keypad you will not notice).
Use a display like 1602 or 2004. See example at 1602 Display. Make sure you order one that has an I2C adapter attached to it (or solder it yourself if you feel comfortable).
After you tested all components separately (make different sketches and save them in case you need them later), you can combine them.
You have chosen a mighty task as your first project. I suggest that you run through as many of the example programs as possible before undertaking your project.
Firstly, I would recommend a matrix keypad instead of the DTMF keypad as it is much easier to use with an Arduino. This will also allow you to use a multiplexer which would use fewer pins.
Alternatively, you could upgrade to a Arduino Mega which has 54 IO pins as apposed to 14 on the Arduino Uno.