There are multiple videos online and the courses on Odoo.com are also very good, and if you follow the respective channels to Youtube you can find much more content.

You can also create a demo installation to try the system, know a bit better how to use it, look up Odoo docs and these forums can also be a good reference to know what kind of issues most users have, and at the same time, discover features you probably never heard off.
This is basically what I have been doing recently, although I've been using Odoo for a few years.

Developing new modules from scratch can also be a good project, to help you understand how the system works, and if you want to go the Database route, install Odoo in your machine, with PostgreSQL, and try to understand how objects are stored, how tables relate to Odoo modules, how users permissions are managed, etc.

If you feel you already know all of the above, look up previous certification exams online, to have an idea of the type of questions you might get, and create your own, to practice, and gather as much knowledge as you can regarding the application and exam questions.