Make sure you familiarize yourself with any journal-specific instructions for peer-review. Some journals will provide peer review guidelines after you accept an invitation to review. Other journals make these guidelines available on the journal's homepage.
Research the journal to get a sense of the journal's content. Refer to the instructions to authors to determine if the paper meets the submission criteria of the journal (e.g. scope, length, etc).
Have a pen and paper ready to take notes while skim reading.