Accept the manuscript for review that meets to your expertise
It is possible that when editor of the journal makes contact with you he may not know your area of specialization. So you are asked to accept only that manuscript that meets your specialization. You will be invited for review and in case of non related manuscript you just decline by sending an email immediately, and then the editor of the journal will make contact with the other reviewers of the journal. If the said manuscript meets your specialization then accept the invitation by mail immediately if you are available to review. 
 Critical behavior
During the review process make sure the clarity of the review. Means review the paper impartially, clearly and honestly focusing on maintaining the quality of the journal. You need to focus on maintaining the quality of the journal by making a critical review report because author will revise the paper according to the suggestions.
Keep the privacy of the manuscript given for review. You are not allowed to use it for your own research work. Also you are prohibited to use the information of the manuscript for your own research work. 
Importance of your review report
Your argument, comments on the paper and the suggestions will be most useful to the editor of the journal to make decision about the manuscript. At this point you are requested to do the review critically and your suggestions should be useful to improve the paper, it should not demotivate the author of the manuscript. You need to review the paper in detail and make suggestions, comments and arguments. 

For review you will be given a deadline. You will have to complete the review process in the given time. Deadlines may be different for different journals because it varies from journal to journal. If you cannot complete the review within time, inform the editor immediately for more time. 

Submission of review report
After completing the review process you need to submit the review report without any delay so that the editor may take the decision about the paper. 

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