Hola Lesley
We use one or the other depending on the context. There isn't one simple rule differentiating them. We use ser in specific situations and estar in other specific situations. We have many specific lessons on these essential verbs. Here are some of the A1 and A2 lessons:
Ser vs Estar in Spanish: Using ser in Spanish (not estar) to describe relationships
Ser vs Estar in Spanish: Using ser in Spanish (not estar) to express origin or nationality
Ser vs Estar in Spanish: Using estar in Spanish (not ser) when talking about locations
I hope this helps. There are more lessons on ser and estar for more advance levels.
Saludos
Inma