Formal:
* Je suis très fatigué(e). (I am very tired) - This is the most direct and literal translation.
* Je suis épuisé(e). (I am exhausted) - This implies a stronger level of tiredness.
Informal:
* Je suis crevé(e). (I am dead tired) - This is a very common and informal expression.
* J'ai les jambes en coton. (My legs are made of cotton) - This is a figurative expression that means you are so tired your legs feel weak.
* J'ai besoin de me coucher. (I need to go to bed) - This is a more direct way of saying you are tired and need to rest.
* Je suis à bout de forces. (I am at the end of my strength) - This is a very strong expression that implies you are completely exhausted.
The best option will depend on the specific situation and your relationship with the person you are talking to.