Learn French: Vous avez tout à fait raison.

In English, you "are" right, but in French, you "have" rightness. The set expression is avoir raison (literally, to have reason). This is similar to how you use avoir for age (j'ai 20 ans) or physical feelings (avoir faim, to be hungry).

What does tout à fait mean in this sentence?

Word-for-word, it translates to something like "all to done," which doesn't make sense in English! However, as a fixed expression, tout à fait is a very common and natural way to say "absolutely," "completely," or "exactly."

Why isn't there an article like la or une before raison?

In French, certain common states of being use avoir plus a bare noun without any article. Just like you say avoir faim (to be hungry) or avoir peur (to be afraid) instead of adding an article, you simply say avoir raison.

Since we are learning about consequences, could I use alors or donc with this sentence?

Absolutely! If you want to say "So, you are absolutely right" as a conclusion to a conversation, you can add them in. You could say Alors, vous avez tout à fait raison (So, you are absolutely right) or Vous avez donc tout à fait raison (You are therefore absolutely right).