My car idle up at the light, then cut off. It will not start at all now when I turn the key. There is no sound. I had the battery checked and it's good. I have a new alternator and I checked all fuses. They said it could be my starter or ignition switch. How can I find out which one it is. 1997 Buick Regal.
There are various factors which can cause the symptoms you are stating.
If your car (1997) has a mechanical ignition system which is highly probable then check your ignition coil and if that is fine then check the spark plug.If your spark plugs work then clean them and check if the problem still exists.(BTW they should be brown in colour if they are dark brown or white then there is problem with your fuel injection system which can cause the spark plug to wear out soon thus causing the stall issue
If your car has an electrical Ignition system then check if the ignition coil pack has an issue and follow the same steps as above.
Check if your Air filter is not clogged up.
Note: I am not 100% sure that the above are the cause , as Paulster said you need to check it thoroughly to know the issue give the above points a try.
You would have to bench test the starter to see if it is the issue, though I doubt it is considering the car cut off them wouldn't restart. The starter will help the engine get started, but once it's started, has nothing to do with how it runs. I would be looking at the ignition switch, but more than likely you'll have to take it somewhere to have it diagnosed. There is just about no way we could help you get it diagnosed over the internet.