Yes, you can use "잘 할 거야" and its variants.
Using polite style,
- You're gonna do a great job / You can do it => 잘 할 거예요 / 잘 될 거예요 / 잘할 수 있을 거예요.
- Don't be nervous => (너무) 긴장하지 마세요 / 긴장을 풀어요.
- Act yourself => 평소처럼 해요 (Do as you usually do) / 자기 (있는) 그대로를 보여 줘요 (Show your true self).