Recently, I cleared my Salesforce Platform Dev I exam and following are the tips from my side to clear the exam:

1. If you have access to a Premier Online Training catalogue or Partner Training catalogue, you can go through the Cert Prep video from Salesforce and it will definitely be a very good starting point and will brush up your skills on the topics given in the exam guide . Link for the video – Certification Preparation for Platform Developer I
2. Go through the questions given in SfdcStudy , Propofs and  Salesforcesteps
3. Please note that some of the answers for the questions given in the above links might not be correct hence double check by googling or post them in comments section and I will try to answer them.

P.S: I found this link from the net today and I believe that it could be useful too https://quizlet.com/133435950/flashcards