If your pan has burnt food stuck to it wash it first in warm soapy water without soaking for too long to avoid blackening of the metal. Then use one of many aluminum cleaning products widely available on the market such as Bar Keepers Friend or Bon Ami, or just simple pantry ingredients like cream of tartar or baking soda by making a paste (mix equal parts powder and water) and a pad such as Scotch Brite to remove the remaining food.
The best way to remove interior discoloration or darkening from the pan is to make a mixture of cream of tartar and lemon juice (1 tsp of each to a quart of water), fill the pan with it, and simmer on low heat until the stains disappear. Let it cool, wash, and dry as usual.