You have the Pass Key system to prevent theft.
The way to do it is you insert the key, try to start, and nothing happens. Wait 3 minutes, with same key, it should start.
You can buy just the tumbler set and this will include a new chip socket and a new key. If your real nice to the parts man at the dealer, he might cut you two extra keys. Be sure to take your original key for comparision so as the dealer can give you the correct chip in the key. Any GM dealer can do this,
I did this to a friend's Cadillac and it cost me a total of $50.