Hi there i have changed my key and the cyl with a used one i had but my 99 comaro dont want to start

That is because your onboard computer system does not recognize the pellet in the key. Your car is equipped with what is known as a "Pass KEY" (PK-1) security system. It uses a resistor pellet in the key to electrically identify itself to the computer system. The key that you are using from your other car does not have the correct resistance to allow the antitheft sytem to enable starting (it "thinks" you are trying to steal the vehicle).

The BEST way to fix this problem is like this:
MOST hardware stores can make new PK1 keys. What you need to do is take the OLD (original) key so they can get the resistance value off of the pellet in the key. Then use the REPLACEMENT (from the other car)key as the template for cutting the NEW key. That way, it will be about the same as having the original key (electrically) that has a different cut to work the replacement lock cylinder (mechanically).

There are other ways to get around the PK-1 system, but this is the best way if you want to keep your Camaro considerably safer among car thieves. The new key will not cost much (a few dollars). The hardest part will be trying to get the hardware store "key-cutting person" to understand what is going on here....use one key to identify the resistor, and use the other key to make the cut.