How to say "Good luck!" in Japanese

Good luck in Japanese is :
頑張ってください Ganbatte kudasai (polite) or 頑張って Ganbatte (casual).

To answer:
Yes, I’ll do my best! ⏩

はい、頑張ります！ Hai, ganbarimasu! (polite)
or うん、頑張る！Un, ganbaru! (casual) in response.