Formal:
* Tout ira bien. (This is the most direct and formal translation.)
* Tout finira par s'arranger. (This means "Everything will eventually work out.")
Informal:
* Tout va bien aller. (This is a more casual way of saying it.)
* Ça va aller. (This is a very common and informal phrase meaning "It will be okay.")
* Ne t'inquiète pas. (This means "Don't worry.")
More empathetic:
* On va y arriver. (This means "We'll get through this.")
* Tout finira par s'améliorer. (This means "Everything will eventually get better.")
The best way to say it will depend on the situation and your relationship with the person you are speaking to.