building an unlock system with bluetooth and mobile phone using arduino
Which bluetooth receiver? Like @user591124, I recommend you take a look a the Arduino Bluetooth board, which includes a BT receiver. If you wnat an external BT receiver, check out search results at Sparkfun.

What software? To program your Arduino, you only need the Arduino software which you can download from the Arduino site.

I believe the real issue here is actually going to be the electronic door lock. A quick Google search turned up the likes of this site. They offer electromagnetic locks as well as control boards. It would then appear that you can hook up one of the Arduino outputs to a relay or transistor, which provide the control board with the correct voltage to lock/unlock the door.

Probably best to put the Arduino on the other side of the door, considering it would be quite easy to break in if you had access to it. :)