In order to participate in the exam you must have fulfilled the following requirements:
-You must be a Genin in order to enter.
-You must have completed at least 3 missions for your village.
and/or
-Your Kage must enter you. (They must think you are strong enough to be a Chunin and it would be in the best interest to promote you)

Exam Guidelines
-You will be promoted based on merit. Not on placement.
-You must give it your all. Hold nothing back.
-The exam will be held just about every 2 months. (in real life)
-It will consists of 3 rounds of tests. They change.

Good Luck !