Identifying the narrator, gender, number and time indicated by a sentence is the first step that must be taken. This is very important to be able to logically conjugate the verbs, and that these have concordance with what is being expressed.
You would use "estar" in a sentence like, "voy a estar con mis amigos" which means "i am going to be with my friends". i don't see how you can use it in the examples you put.
