Myths can be stories, which explain some natural phenomena or culture-related stories which give society a sense of identity. You need to decide which one of them you are more interested in. They are sometimes explain origins of specific group.
You need to determine your motives when writing a myth:
Should it provide background story for a specific piece of fiction or reality?
Or do you wan to make others believe in something that is not true
Characters should fit into archetypes, which are commonly recognizable and understood. Myth should be short setting well set, but don't be too specific with details, everyone should be able to adapt the setting to where they live.
