If you're not handy, have someone clean it up for you. If you are handy, take the burner off and run a rope through the fire tube. It's just a U-tube and it should be easy to get the rope through. Then tie a rag onto the rope and pull it back and forth through the tube to make sure there's no debris in there blocking it. Put the burner back on and fire it up.
Buy some screens made to go over all the holes in the cover for the water heater. I had to put a thin washer under the catch to keep those darn wasps out! While you'er at it, buy screens for your refrigerator and furnace so they don't mess them up too!