c'est pourquoi

Meaning

Announces the consequence as an explanation of what you are about to do.

Position and punctuation

at the start of the sentence · no comma

Example

Je n'ai reçu aucune réponse depuis trois semaines ; c'est pourquoi je me permets de vous relancer.

I have had no reply for three weeks; that is why I am writing again.

In the DELF

the B1 letter · the B2 essay

The trap

It is the LETTER connector: it justifies the step you are taking. No comma after, since pourquoi already belongs to the next clause.