Here are some ways to clean aluminum cookie sheets:
Baking soda
Sprinkle baking soda on the cookie sheet and spray with hydrogen peroxide until the baking soda is wet. Let it sit overnight if possible. Baking soda is gentle and won't scratch or discolor the finish. You can also make a paste of baking soda and water and use a soft-bristle brush to clean stains.
Baking soda, vinegar, and salt
Sprinkle baking soda, then salt, then spritz with vinegar. Scrub after five minutes.
Borax and warm water
Borax can remove stains, sticky grime, and burnt food.
Cream of tartar and vinegar
Mix cream of tartar with white vinegar and soak the cookie sheet for about eight hours.
Dryer sheet
Put a dryer sheet in the bottom of the pan with some dish soap and boiling water. Let it sit for 30–60 minutes. The dryer sheet can help break down grease and stains.
You can also soak the cookie sheet in soapy water and then clean with a non-abrasive sponge. When you're done, rinse thoroughly with warm water.