Professional Scrum Developer I (PSD I) exam is comprised of theory and concepts from SCRUM Development methodologies and so the prerequisite for this exam is that we need to have knowledge of SCRUM along with the development methodologies. This exam is not so easy as the PSM I and you need to have very good development experience.
Here are the things to be taken care of while preparing the exam:
1. Read SCRUM guide available in scrum.org
2. You should understand the TFS if you are a Microsoft Developer and have knowledge about branching and merging in TFS. You should know when/how to merge and branch the source code.
3. You should understand the Test Driven Development concepts and Acceptance Test Driven concepts. It is advisable to have practical knowledge on the same. In case if you have concepts try to implement a simple example to understand the concepts better.
4. You should know the Build Server and how it works.
5. Most of the times the questions are related to your development experience and often needs judgment in selection of answers.
6. Also you should know about Code Coverage and Unit Testing. Also Code Analysis and Code Metrics. If you do not use these very often in your projects, it is advisable to check the same using Visual Studio.
7. Finally take practice exam (Open Assessment) available in scrum.org. But do not assume you will get more questions from the same. Most of the questions are related to real time experience.
If you are new to everything it is advisable to take a preparation session or course before attempting the exam.