Summarize:

Not every baking sheet will respond well to a given cleaner. Before applying any cleaner to your sheet, test out the cleaner on a small portion of the baking sheet. Make sure there is no discoloration or damage. Ammonia is toxic, so wear protective gloves when handling it. You should also handle ammonia in a well-ventilated area to prevent breathing in the fumes. If possible, handle ammonia outside. In the future, line your sheet with parchment paper before using it. This will protect the pan from grease and food residue. After using the pan, you will simply have to remove the dirty parchment paper.
Do a spot test before applying any cleaner. Take safety precautions when using ammonia. Use parchment paper when baking.