Formal:
* Bonne chance, notre ami(e). (This is the most literal translation and works well in most situations)
Informal:
* Bonne chance, mon ami(e). (This is more familiar and can be used with friends or close acquaintances)
* Bon courage, mon pote! (This is a more casual and friendly way to say "Good luck")
* On te souhaite bonne chance! (This is a common way to say "We wish you good luck")
More specific to the situation:
* Tu vas réussir! (You're going to succeed!)
* Je te fais confiance! (I have faith in you!)
* Je suis avec toi! (I'm with you!)
Remember to use "ami(e)" for a friend, "mon ami(e)" for a close friend, and "mon pote" for a very close friend or buddy.