I leave you a link to a website where they try to explain the uses of the verbs ser and estar:

http://www.studyspanish.com/lessons/serest1.htm


I'm going to put some examples of sentences that change their meaning depending on wich verb we use.

Soy listo = I am smart.
Estoy listo = I am ready.

Eres alto = You are tall.
Estás alto = You are in a high place. You could said this to a child whom has grown a lot since the last time you saw him.

Es una cena fría = The dinner consist in salads, and other raw meals.
La cena está fría = It was a hot meal but it got cold. possibly because the people were late.

Es gordo = He has been always fat person.
Está gordo = He is fatter now than before.

When you talk about colours you must use ser for things with a fix colour.

El coche es verde = The car is green.
La jirafa es amarilla = The giraffe is yellow.

But if something usually change his colour you would use estar.

Estas amarillo = Your skin is yellow maybe because you are ill.
El botón esta rojo = It could be red or green and in this moment is red.
El campo esta verde en primavera.

I'll be glad if you correct my mistakes with english.
Don't hesitate to ask any question.
Try to make sentences with these verbs.