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.