You have to use "c'est" when there is an article (une, un, le, la, or l'), a possessive (mon/ton/son, ma/ta/sa, notre/votre/leur) before the noun, or before a possessive pronoun (le nôtre/le vôtre/le leur/la nôtre/la vôtre/la leur).
Ex: You have to say "C'est mon garçon. C'est le mien", NOT: "Il est mon garçon. Il est le mien."
Note: you can use "Il est" before an adjective (ex: "Il est riche." = "He is rich."), before a noun of occupation with no article (ex: "Il est avocat." = "He is a lawyer."), or before an adverb (ex: "Il est tard." = "It is late.").
The same rules apply for the feminine singular pronoun "elle" (except for the adverb case).
You have to use "ce sont" when there is an article (des or les), a possessive (mes/tes/ses, nos/vos/leurs) before the noun, or before a possessive pronoun (les nôtres/les vôtres/les leurs).
ex: You have to say "Ce sont nos garçons. Ce sont les nôtres", NOT: "Ils sont nos garçons. Ils sont les nôtres."
Note: you can use "Ils sont" before an adjective (ex: "Ils sont riches." = "They are rich."), or before a noun of occupation with no article (ex: "Ils sont avocats." = "They are lawyers.").
The same rules apply for the feminine plural pronoun "elles".