HI,
Prepare yourself with for exam by gaining functional knowledge across all the main modules and give a try. You need to have a minimum of 70 % marks to pass the exam.

For the preparation:

* Odoo documentation
* Odoo E learning Videos
* Follow the latest updates in the newer versions

Also see:  How to Get Odoo Certified

Once you done the preparation go for this sample test: https://www.odoo.com/slides/odoo-certification-sample-test-50

Thanks