1. Fristly you go through all the course and tutorials available in odoo for different modules -https://www.odoo.com/slides/all

2. If possible get enterprise version or else download community version and get some handson experience on it.- https://www.odoo.com/page/download

3. If you get the confidence on the same, then go to odoo.com certification..check odoo 18, add to cart and buy the link and then give the certification test..-https://www.odoo.com/slides/odoo-18-functional-certification-442