Formal:
* C'est votre anniversaire aujourd'hui. (This is your birthday today.)
* Joyeux anniversaire! (Happy birthday!)
Informal:
* C'est ton anniversaire aujourd'hui. (This is your birthday today.)
* Bonne fête! (Happy birthday!)
* Joyeux anniversaire! (Happy birthday!)
You can also use a sentence like:
* Aujourd'hui, c'est ton anniversaire. (Today, it's your birthday.)
The choice of which phrase to use depends on your relationship with the person and the overall tone of the conversation.