How do you know which of 'ser' and 'estar' you need to use when saying 'to be'?

You're right that both of these verbs mean 'to be' in Spanish. They are used specifically in different contexts, so it's possible to learn these contexts and use both verbs correctly rather than guessing at which to use! In English we use the verb 'to be' to mean all sorts of different things - both to describe the condition of things, and the essence of things. The 'condition' of something is a non-permanent state, whereas the 'essence' of something is permanent. While this distinction might not seem like much in English, in Spanish it means that you use a different verb. 'Estar' is used for non-permanent states of being - most commonly locations, and 'conditions' (such as emotions and moods). 'Ser' is used for permanent states of being - most commonly appearances and origins. For example, you would use 'estar' in the case of tiredness - 'el niño está cansado' (the child is tired - a condition), but you would use 'ser' in the case of height - 'el niño es pequeño' (the child is small - an essence, even if it might change!). Similarly, while you would use 'estar' for locations - 'el niño está en la cocina' (the child is in the kitchen - this is temporary), you would use 'ser' when describing origin - 'el niño es de Londres' (the child is from London - this won't change).