There is a chip in embedded in the key. That is the ignition activation chip. It has to match the code in the ECU. If a key gets inserted into the ignition that doesn't match that code, it disables the car, and won't start. If you don't have the remote fob key, then I would go to a Lexus dealership, and have them do it for ya. There should be a code registration along with the owners manual, and an emergency key and case. They can program a new key for you.
And here is a free tip for ya. When they program the key, and set up the remote functions for you, you can have them change the lock and unlock sequence for you. For example, on the remote key, if you press unlock once, the driver door unlocks, if you press it again, all the doors unlock. You can have that changed to where when you hit the button, all the doors unlock the first time. Or the driver door and the rear passenger door. Or any combination you want. I have it so all the doors unlock with the first press of the button. makes it easier for the wife to get in and out of the passenger side.