Formal:
* J'espère que vous allez bien. (This is the most formal and literal translation)
Informal:
* J'espère que tu vas bien. (This is more casual and used with friends or family)
* Comment vas-tu ? (This literally means "How are you?" and is a common way to ask someone how they're doing)
* Ça va ? (This is a very casual way to ask "How are you?")
Other options:
* Je te souhaite une bonne journée. (I wish you a good day.)
* J'espère que tu as une bonne journée. (I hope you're having a good day.)
The best option to use will depend on your relationship with the person you are speaking to.