The first time you peer review a paper can be a stressful, but educational, experience. The article below breaks down the overall peer review process. The guide was written for biology; but 99% of it is useful for astronomers as well. I like that the peer review process is placed into a larger context and they discuss the motivations for why people peer review in the first place.
There are also other links listed on the Peer Review Wiki page. What resources have you found useful in guiding your referee report philosophy? Put ‘em in the comments below and/or update the wiki page directly!