So, first thing I did was to get the LCD and keypad working together.
For this I pretended that it was some sort of arm/disarm (or entry/exit) thing.
My code for this part is at: http://pastebin.com/YndLneqm.
Getting the LCD wired up was tricky as most wiring diagrams for it don't show the last two pins wired up and these are required for the back light. Check my snazzy Fritzing wiring diagram to see how I wired up my LCD screen, pot (for adjusting brightness) and keypad. The pins for them all are also mentioned in my code.
NOTE: pin 1 isn't used as whenever I wired something to pin 1 I experienced weirdness. Not sure why. I expect it's something to do with the pins data uses?
If you don't have the keypad.h library you'll need to grab it from here: http://playground.arduino.cc/code/Keypad