example 1: Voy a ser mejor alumno. Ser was used in this sentence because it means going to be and it makes sense in this context
(another example could be "Puedo ser eso?" if the first one isn't good enough)
example 2: Voy a estar en mi casa este sabado. Estar was used in this sentence because it can mean present but in most cases the future which would be "will be" or "going to" which makes sense in that context.
Sorry if this is bad I tried to explain it the best I could!
