You will use ESTAR to describe how you are at the moment and/or where you are. For example: Está muy bien// I am very good. For that I would use ESTAR.
You will use SER for more long term things such as personality, where you are from, an occupation, or describing someone or thing. For example: Soy de Washington, D.C.// I am from Washington, D.C. or El hombre es muy simpático.// The man is very nice.