Grosso motto, these guidelines are fairly helpful.
-C'est is used whenever a demonstrative pronoun (this, that, these, etc) is called for. C'est bien! To pluralize, do not use Ces sont: rather, use ce sont, as in Ce sont de jolies fleures.
-Il est is used in expressions of time (Il est 14h35, etc).
-Il est is used in all manner of fixed expressions, where Il means it: Il est important que, il est l'heure de, il est certain que, il est douteux que, etc... Gramatically, this is the 'correct' way to say these things. However, in common parlance, C'est can be inserted into any of the expressions of this last subset: C'est important que, C'est l'heure de, C'est certain que, C'est douteux que, etc...
This is not exhaustive, but may help a bit!