Mobile controlled lock...

I want to build a mobile phone controlled electronic lock...what would be my basic requirements??

You would need a cellular modem a motor/solenoid or other actuator to turn the activate the lock (and whatever mechanical parts needed to make the lock stay locked, or unlock when power fails depending on what you want),

and a microcontroller reading the modem signals and controlling a relay or other switch to control the locking actuator.

Have a look here, similar to what you're looking for:
Ashish's Programming Journal: Cell Phone Controlled Door Latch