Reviewing a paper someone else has written can be an intimidating process, especially the first few times you are faced with it. It can be difficult to think critically about someone else's research and then write a thoughtful but succinct review
In addition to your postdoctoral advisor, below are resources that can help you with the process of writing a peer review (APS does not endorse or assume responsibility for the information posted on these web sites).
Resources from APS Archive