While in English both of these words translate into the same thing, "To Be", in Spanish they mean slightly different things and can change the meaning of phrases if you use the wrong one. So lets take a look at when to use each one.
As you can see from the chart there are some similarities between the two forms. For example both forms of I am end in "OY", both forms of we end in "MOS", and both both forms of they and you all end in "N"
Be sure to note all the accent marks in the forms of Estar, these really do matter and will change the meaning of the word if you leave them off.
Ser is used in normal and regular situations, situations that may change but are usually considered to be pretty stable. These would be the EVERYDAY situations. A handy mnemonic device is DOCTOR, keep reading below to see an explanation.
Description- a physical descriptions or what they look like.
Marco es muy alto. (Mark is very tall.)
Origin - where someone is originally from, or where they grew up.
Yo soy de Montana. (I am from Montana.)
Characteristic - what their character or personality is like.
Nosotros somos muy inteligentes. (We are very smart.)
Time - when things are taking place.
Son las nueve. (It is 9 o'clock.)
Occupation - what someone's job is.
Tú eres un escritor. (You are a writer.)
Relationship - how are they connected to others.
Ella es mi hermana. (She is my sister.)
Estar is used in changing situations, situations that are based more on current conditions than a set standard. These would be the TODAY situations. An easy way to remember this is the following rhyme.
For how you feel,
and where you are
always use the verb ESTAR.
Here are some examples
Feelings [How you feel]
Ella está triste. (She is sad.)
Ellos están cansados. (They are tired.)
Locations [Where you are]
Nosotros estamos en casa. (We are at home.)
Tú estás en la clase de inglés. (You are in English class.)
Remember, if it doesn't use one of these verbs then it will use the other, so if nothing else remember the rules for estar, if it doesn't fit into them then you know you have to use ser.
Use the following video if you didn't quite get the explanation. He also makes some additional points, such as how the sentences meaning can change by changing the verb used.
The frog website we use in class also has grammar practices.
This has some good practice as well as good explanations.