Changing Object Data on Creation

Hello,

I started to Script some days ago, and already made a small script.
But I have some problems, I want it to be possible that if there's created a new Object, f.e. a box, which isn't created by the script, but by the user, that it f.e. change its Material or wirecolor automatic. Is this possible and how? I googled and found nothing about this.

I hope you can help me