I think if you are already working on a real Sitecore project then you have a good amount of knowledge to pass the certification exam because most of the questions are scenario-based and require you to think through a problem instead of reciting something from memory. However, if you have just started learning Sitecore and want to appear for certification without prior working experience on the Sitecore project then also need not worry, there are plenty of resource out there, which should be helpful to thoroughly prepare you for the exam.
Sitecore eLearning Platform: On the Sitecore e-learning platform (learning.sitecore.com), you can enrol yourself in the Sitecore Experience Platform—Technical Product Solution which consists of the following Collections: Developer’s Fundamentals, Sitecore Experience Accelerator, Helix, Java Script Services, and System Administrator. These collections teach you many of the fundamentals of Sitecore, the platform, Sitecore terminology, patterns, and implementations.
Instructor-led online training: On e-learning platform, you can register for this course Sitecore Experience Solution Developer 9.1. The course focuses on MVC design patterns, Sitecore’s Presentation Architecture, the creation of dynamic presentations, the utilization of the Sitecore API, interactions with visitors to your site, the implementation of search-driven capabilities, the use of Sitecore Security Architecture, and Sitecore workflows. The purchase of a seat in the Sitecore Experience Solution Developer 9.1 course includes the option to take the Sitecore Developer certification exam.
Additional resources: Have a look at this thread which will provide you with a list of resources to start with Sitecore learning.