Hi Shakil

the best to prepare your certification is to do the odoo tutorial

https://www.odoo.com/slides/all/tag/odoo-tutorials-9

they contain also  Quiz that can prepare you

Since odoo certification is issued by odoo, you have tu purchase it from odoo, and yes they are costs on behind

like the v17
https://www.odoo.com/slides/odoo-17-certification-331

For more recent information, please see https://www.odoo.com/event/2622/track/2064 (to prepare for) and https://www.odoo.com/slides/all?search=cert (to take) the Odoo Certification Exams for each version.

Hope this help you