Tu as tout à fait raison.
Here's a breakdown of the phrase:
* Tu: You (informal, singular)
* as: Have (verb "avoir" in the present tense)
* tout à fait: Absolutely, entirely
* raison: Right, reason
Other possible translations:
* Tu as parfaitement raison. (You are perfectly right.)
* Tu n'as pas tort. (You are not wrong.)
* C'est exact. (That's exact.)
The best translation will depend on the context and the level of formality.