HI,

* you can follow the odoo official documentation and learning videos. Along with it, you should have experience in using the system.

Documentation: https://www.odoo.com/documentation/16.0/

E learning: https://www.odoo.com/slides

* yes, there is a sample certification that you can try for free.

Sample certification: https://www.odoo.com/slides/odoo-certification-sample-test-50

* no, official certifications are paid and there is some free certificates provided by third party,

Thanks