Replacing car keys?

I was having a single key for my Nissan primera car when it got lost.But I removed the switch and took it to a local lock smith to cut another key.They did it and the cut key was able to open the doors and turn on the switch. The engine will come on and went off within seconds. It was then I realize that the original key has chip that is design to work with the immobilser. What do I do to get another key? Or help me on how to by pass these system so that I could get it working with any other key.