Formal:
* J'ai la paix intérieure. (This is the most literal translation, emphasizing the feeling of inner peace.)
* Je suis en paix. (This emphasizes the state of being at peace.)
* J'ai l'esprit tranquille. (This literally means "I have a quiet mind," emphasizing the absence of worry.)
Informal:
* Je suis serein(e). (This means "I am calm," implying peace of mind.)
* Je suis tranquille. (This means "I am relaxed," also implying a lack of worry.)
* Je n'ai aucun souci. (This literally means "I have no worries," emphasizing the absence of problems.)
The best choice for you will depend on the context of your sentence and the level of formality you need.