Generally estar means: to be in the state of (in the state if sadness in the
state joy , illness, Kansas
, etc.)
Ser is less transient and more of an inherent characterisitic. Sometimes ser and estar can be interchanged but the meaning changes. Ella es rubia. Ella está rubia for example.
With that in mind there a also genuine solid rules which you need to know and practive. You can always pull them up on the internet.
Here's an example:
http://www.spanishdict.com/answers/100040/ser-and-estar
Most people who are not native Spanish speakers make mistakes with ser
and estar, and native speakers will, for the most part, understand anyway.