Formal:
* Tout ira bien. (This is the most literal translation and means "everything will be fine.")
* Ça va aller. (This is a bit more reassuring and means "it will be alright.")
Informal:
* Ça va rouler. (This is a slang expression meaning "it will be smooth sailing.")
* Y'a pas de soucis. (This means "no worries" and is a very casual way to say it will be okay.)
* On va s'en sortir. (This means "we'll get through it" and is a good way to show support and reassurance.)
The best choice depends on the context and your relationship with the person you are speaking to.