I do not know much French and shall await SraJMcGin with the definitive answer.
In the meantime, let me suggest this web site with rules for making the selection:
Use C'est for the first one
Thank you for using the Jiskha Homework Help Forum. Let's look at your first question first! It doesn't seem to be a big difference between C'est and Il est. Here is as easy as I can make it:
Before a modified noun: C'est
C'est un garçon intelligent. = He is an intelligent boy.
Ce sont mes cousins. = They are my cousins.
BUT unmodified: Il est
Il est professeur. = He is a teacher.
In addition, other uses of C'est (ce / c' = it, he, she, this, that, they, these, those - used only with the verb être. Then "Ce" replaces "il, elle, ils, and elles" with the modified noun. (see above)
Before a proper noun. = C'est
Qui frappe? C'est André. = Who is knocking? It is Andres.
Quelle est cette ville? C'est Paris. = What (Which) city is that? That is Paris.
Before a pronoun: C'est
Qui parle? C'est moi. = Who is speaking? It is I (I'm the one).
C'est celui de mon père. = It is my father's.
Ce sont les miens. = They (These, Those) are mine.
Before a superlative: C'est
C'est le meilleur du monde. = It is the best in the world.
C'est le plus beau de la classe. = He is the most handsome in the class.
Hopefully that will clarify it for you.
Ijust read drwls' response and that website is excellent. You won't even need my response!