Sorry that this has to be kind of a generic answer, but here goes. A rain cap should be inexpensive. Go to Home Depot or Lowes and ask for the cheapest rain cap they have. It should be a simple conical shape and should just slip on your flue pipe (typically 4" diameter) and secure with two or three sheet metal screws. You might look at your neighbor's houses and see what they have. There are some more complex ones if you have trouble with windy conditions causing puffs of wind to come down the flue pipe and possibly blowing out the pilot light.
