C'est versus il/elle est

It sounds simple enough, but let me warn you: if it's easy to understand the rule, it's complicated to apply it. It's already hard enough for an English speaker to refer to a thing by him or her, yet it is even harder to use c'est + a person. It sounds in English like you are saying it's + person... very very bad...

You need to train a lot on this concept before it becomes natural to you.

A - General Rule
In English, you say he is a friend, he is charming. So you use he is + noun (a friend) as well as he is + adjective (charming). Well, in French, we use 2 different constructions.
il/elle est + adjective (plural ils/elles sont)
il est grand, elle est blonde, ils sont amusants.
c'est + noun (plural ce sont - but we often use c'est + plural although it's a mistake...)
C'est un ami, c'est mon mari, c'est cette voiture, ce sont mes cousins.
Examples:
Le Père Noël: c'est un homme gentil. Il est gros. Il est généreux. C'est un personnage magique.
Les Jackson 5: Ils sont célèbres. Ce sont des chanteurs. Ils sont mignons et talentueux.

You will never have "il est un / elle est une / il sont des". It should always be "c'est un, c'est une" with a strong liaison, "ce sont des". The only exception I know is "Il était une fois" which means once upon a time.

Now, some particular cases...

B - Particular Cases
1- Adjectives that come before the noun
As you know, some adjectives come before the noun; grand, petit, joli, jeune, vrai, bon, mauvais...
So what should you do when you have a sentence with one of these adjectives? Well, you have to see if the adjective is followed by a noun, or if it is alone. If there is a noun, use c'est.

C'est une belle voiture.
Elle est belle.

2 - Nouns of profession, nationality, religion
Nouns of profession, nationality, religion... can be used as adjectives - only if there is no other adjective describing it.

So, when it is used as an adjective, use il/elle est.
Il est français. Il est medecin. Elle est juive.

But you can also use it as a noun. In this case it needs an companion word (article, possessive or demonstrative adjectives....)
C'est un Français. C'est un medecin. C'est une juive.

Now, if you wanted to say "he is an intelligent Frenchmen", Frenchmen cannot be an adjective because you have another adjective there. You have only one possibility ; c'est un Français intelligent. You cannot say Il est français intelligent...

3 - C'est + adjective masculine singular
To make a live comment, react to something, share your experience, we use the construction c'est + adjective masculine singular. It's your emotion that comes through, not a specific description.
C'est beau ! c'est bon ! c'est chaud !

Watch out that the adjective cannot be in another gender/number ; c'est belle is not possible, even if you are looking at "la mer"(the sea). The construction demands a masculine singular adjective.
This construction is also use to make comments about something as a category:
la mer, c'est beau!
I am not talking about one sea or ocean in particular, but all the seas in the world.

So, let's imagine some scenarios:

You are talking about the Mediterranée : you could say. "Comme elle est belle, la mer Méditerranée. Elle est bleue, elle est transparente. C'est vraiment une belle mer."

You are standing in front of the bay of Cassis, and are overwhelmed by the beauty of the landscape. You say "Woah... c'est beau!!" - it's your emotion speaking, you are talking about the sea but also the light, the rocks, the feeling you are having. Kind of "how gorgeous" in English.

So now, let's have some examples.

C - Examples
Voici mon ami Pierre. C'est un homme charmant. Il est grand et brun. Il n'est pas marié. C'est un bon musicien. Il n'est pas riche, mais il est passionné. C'est un rêveur, il est un peu timide, mais c'est un bon copain.
This is my friend Peter. He is a charming man. He is tall and has brown hair. He is not married. He is a good musician. He is not rich, but he is passionate. He is a dreamer, he is a bit shy, but he is a good friend.

Live situation:
C: Elles sont comment vos lasagnes ? How are your lasagnas?
S: Ce sont des lasagnes à la viande. Elles sont riches et copieuses. They're meat lasagnas. They are rich and copious.
C: Est-ce qu'elles sont chères ? Are they expensive?
S: Non, elles ne sont pas chères. No, they're not expensive.
C: Les lasagnes, c'est bon ! Lasagnas are tasty!
S: Oui, et nos lasagnes, elles sont vraiment délicieuses. Yes, and our lasagnas are really delicious.

La mer est bleue. Elle est verte. Elle est violette. Elle est noire. C'est un élément changeant. C'est toujours beau, la mer. Mais la mer des Iles grecque, elle est particulièrement belle.
The sea is blue. It is green. It is violet. It is black. It's a changing element. The sea, it's always beautiful. But the sea of the Greek islands, it's particularly gorgeous.