You should start with Exam 1 OR Exam 2.
Exam 3 would be the worst exam to choose to start off with out of the 4.
Read the handbook, do some questions, go over the syllabus guideline which you can find on the PRMIA website and go over your weak point. Repeat.
Use online resources like RiskPrep and Kesdee.
Also just a heads up, I’ve received emails from PRMIA lately confirming they are in the process of updating their syllabus (no dates have been confirmed) so just bare that in mind when you start studying for a particular exam.