In Spanish, both 'ser' and 'estar' mean 'to be,' but they're used in different situations. The key is that 'ser' describes permanent or essential characteristics, while 'estar' describes temporary states, locations, and conditions.
Steps
Use 'ser' for permanent characteristics like personality traits, physical descriptions, nationality, profession, and relationships (e.g., Ella es inteligente - She is intelligent).
Use 'ser' for telling time, dates, and where events take place (e.g., La fiesta es en mi casa - The party is at my house).
Use 'estar' for temporary states and emotions like feelings, moods, or current conditions (e.g., Estoy cansado - I am tired).
Use 'estar' for physical location of people and things (e.g., El libro está en la mesa - The book is on the table).
Remember that some adjectives change meaning depending on which verb you use (e.g., 'ser aburrido' means to be boring, while 'estar aburrido' means to be bored).
Worked example
Compare these sentences: 'Mi hermana es alta' (My sister is tall - this is a permanent physical trait, so use ser) versus 'Mi hermana está enojada' (My sister is angry - this is a temporary emotion, so use estar). Also, 'Soy de México' (I am from Mexico - origin uses ser) but 'Estoy en México' (I am in Mexico - current location uses estar).
Remember
Think of ser as describing what something IS essentially, and estar as describing how something IS right now or where it IS located.