Exams have this crazy and weird power of shaking you to your nerves.
Do not worry. here are a few tips to help you:

Manage time efficiently
Start prepping early.Make your own notes.
Talk it out with someone.
Visualize success.
Scan each page of your question sheet to analyse properly with a stable mind what you are sure about and what you are not.
Until the end, go back to the answers to review them.
Breathe deep with faith.