Before the exam practice working with classmates or practice in front of a mirror, to evaluate yourself.
Maintain your self-confidence and composure.
If you find yourself anxious, take several slow, deep breaths to relax.
Don't talk about the test to other students just before entering the room: their anxiety can be contagious.
Correct yourself if you make any mistakes.
Keep eye contact.
And remind yourself that you are well-prepared and are going to do well.
The British Council has published a set of videos explaining what you should and shouldn’t do during a speaking test. Please watch them and take notes, the advice given will probably be useful for you in the future.