I am sure you will get lot of responses from other users. I can only highlight mine tips.
1. Check curiculum of the exam. In past you could find detailed document for DA 100 exam (which is basically older name for PL 300). This is from last year, but maybe you will find more recent: https://query.prod.cms.rt.microsoft.com/cms/api/am/binary/RE4qlRu 2. Once you check the exam topics focus on those which you really don't understand or even never heard of. To learn more, take a look on MS Learning Paths https://docs.microsoft.com/en-us/learn/browse/ 3. Be sure you have some PBI experience. I would say at least several months with reporting and some experience with PBI service. Ofcourse you can apply without any experience, but then it is gonna be very difficult. 4. Prepare that the exam is quite demanding in terms of time. I personally had a problem understanding all the text at the end since it was just too long and my concetration dropped.