How do I know when to use SER or ESTAR

Both SER and ESTAR mean TO BE in Spanish but are used in different situations. Ser gives a more permenant meaning than estar. Here's an example: está enferma and es enferma both mean she's ill literally. You need to use the first one (estar) as it's a current state and not a permenant one 

The easiest way to remember it is to remember it is to use these little tricks:

For SER: The DOCTOR numonic

Description - Es imposible 

Occupatoin - Soy profesora 

Characteristic - Eres inteligente 

Time - Es la fin de semana 

Origin - Mis primos son italianos 

Relationship - Pilar es mi madre 

And for ESTAR there's a little rhyme: how you feel and where you are, that is when you use estar. EG. 'Estamos en colegio' or 'Estoy contenta'.

Also don't forgot to use ESTAR + present participle if using the continuous present tense. EG: estoy corriendo (not soy corriendo)

And that's it :) just remember the numonic and the rhyme and you can't go wrong.