An advice which seems to give some insight for this would be: if you can substitute the it in the English sentence by a this or that then it is probably a ce, cela and other variants of them which is needed in French. You have to be aware that the it pronoun (neutral gender) does not exist in French, that is why it might be tedious to translate sentences such as "This mountain is so big. It should be at least 3,000 meters high.", for which you have to know the gender of the word montagne in French.
More generally, ce, cela (cet, cette, ceux, etc.) are demonstrative pronoun which means they refer to something you already mentioned or something which is obvious according to the context. Whereas il and elle are just the French equivalent of he and she, plus the expression there is/are in English which translation is il y a in French.