How to celebrate Halloween?

Halloween is coming soon. Normally How do Americans normally celebrate (don't get confused with celibate!) this holiday? I know the last day of October is Halloween, and see many people decorate with painted pumpkins outside their houses, besides together with other white clothing/sheets band and human skeletons. What do these mean for Halloween? I heard that many children will go around the neighbours, for asking for candies/sweets/lollies ('trick or treat'). So do we need to prepare by having some candies at hand?