To act as a reviewer you need to make sure the article you have been asked to review truly matches your expertise. Typically, editors determine your expertise based on your published work. You need to also make sure you can review the manuscript within the set timeframe. Reviewing papers can be time consuming. Editors are always looking for  experienced reviewers. Typically either the journals you submitted work to or those you have papers under review in will end up sending you invitations to review papers. Do not accept unless you're confident you master the topic. There are no consequences for refusing to review a paper, however your reputation is on the line if you fail to deliver a good review.

http://www.elsevier.com/reviewers/reviewers-update/archive/issue-10/how-to-become-a-reviewer

The link Nadine shared contains good information.  To be chosen as a reviewer, you need to write articles.  Editors will look at articles previously published in their journal, and may look at published authors in other journals with similar topics.  Many journals will ask you, when you first register, about your areas of expertise.  Your answers go into a database that they can use to find appropriate reviewers.

Dear Nadine and Michael,

Thanks for your answers. I am acting as reviewer for some journals. No where I register myself as reviewer. Just acting on invitations. Recently I am getting more number of invitations from journals. I wonder where from they get the details. So its that they view our publications and conclude our expertise. 

There were actually two questions. :-) [The minimum requirement that I think should be rule is not often the criterion by which journal editors select reviewers.]

A natural requirement, I think, is that a reviewer should have some experience in being reviewed her/himself. That role is not kept by journals, I think, and I was myself asked to perform a refereeing duty quite early on in my career, at least long before I had something published. A reviewer should have some education in the role and responsibilities of a reviewer - someone with experience should explain how refereeing is done, and how it should NOT be done. It is a very important duty, and is NOT to be taken lightly.

Bad refereeing implies a very bad reputation in several ways and places - it is bad for the author, and it would be bad for the journal if a bad paper is published. 

It is good that the refereeing duties are described by journals. What I do NOT like at all, however, is when a journal stipulates that the reviewer should to try persuade the author(s) to add articles that are published by the same journal or publisher - that's plain awful practice.

But the best general rule, I suppose, is that the reviewer should check for correctness, for good writing, and for the fact that the paper is within the scope of the journal. And that it has not been done before, of course. (That one requires a lot of knowledge of the field, whence any quite young referee should consult more experienced researchers for advise.)

Thats really very nice Dr.Michael Patrikkson. You have discussed a lot on practical difficulties and happenings.

Even I come acrossed such a situation (as a author) where a editor who acted as a reviewer too insisted to cite his own articles in number.  

But i didnt face any problem as a reviewer to insist authors to add publications from the specific journal.

Reviewers must have a terminal degree in their field OR 20-30 years of expertise in fields which do not award terminal degrees. Additionally, Reviewers should be expert in any field they volunteer to review. Reviewers need to submit a CV or resume to the Board of Editors. The Board of Editors will assess the Reviewers qualifications and email them a decision.

http://jnegres.org/author-and-reviewer-qualifications/

To act as a reviewer you need to make sure the article you have been asked to review truly matches your expertise. Typically, editors determine your expertise based on your published work. You need to also make sure you can review the manuscript within the set timeframe. Reviewing papers can be time consuming. Editors are always looking for  experienced reviewers. Typically either the journals you submitted work to or those you have papers under review in will end up sending you invitations to review papers. Do not accept unless you're confident you master the topic. There are no consequences for refusing to review a paper, however your reputation is on the line if you fail to deliver a good review.

Peer reviewers play an important role in determining which research is to be published. Therefore, they are judges and they should be appointed based on their knowledge, prior experience, and a historical demonstration of impartiality and fairness.

Apart from PhD, the following is required as essential minimum requirements to act as a reviewer for research publications.

Dear All,

There are independent (free lance?) and by the editors well known reviewers (these may have a close relationship with the editors that is why these people are often asked to prepare reviews). Both types should be professionally distinguished and honest with a long experience and perhaps with highly special knowledge. In normal cases, editors do not want to influence the reviewers’ opinion. If they want to do it – what may occur – the second type (mainly if there is a subordinate relationship between editor and reviewer) can be easier manipulated.

For reviewer many factors will be considered by the Editor. In many Journals they will see your related publication or will ask the CV. Its also depend on relation with Editorial team/ Editor. Many time you have to be punctual within Journal's time frame. It is really a difficult task to review a paper and put a suggestions/ comments on that particular topic.  It is only justified that field of work should be matched or reviewer should capable to do the review. Many time the topic is different or we are busy so we have to refuse the same at that particular paper..

Hope Dr Sanmugapriya Ekambaram, you are doing well that is why you are receiving review of many Journals.

The essential minimum requirements to become reviewer:

1. Good research publications in a particular field

2. Specialization in the area

3. Good research experience

4. Designation does not matter

Some times strong CV in the related field attract editors to select the reviewers. The data reflected in the published research is also taken into consideration. I also agree with the opinion of Lawrence Broxmeyer on inside polytics. I would like to add a point here; a reviewer should not only provide comments for the articles to be accepted but also all the necessary points required to support the article (those which will be rejected) for its publication.

Minimum requirement is ---- one must be doing research.

Think minimum requirements to act as a reviewer for research publications include:

1) Content Knowledge - i.e. you are subject matter expert & have relevant experience in certain knowledge domain e.g. cloud computing, big data analytics in IT related research.

2) Research Methodology - i.e. you have the relevant skillet to evaluate the article's methodology e.g. with only quantitative research skillset, the reviewer can't review articles which are qualitative or mixed method related in social science research.

3) Active Researcher - a reviewer candidate might not be suitable if s/he is inactive researcher / lost touch of a particular research trends / progress, not attending relevant research conference for sometimes etc.

4) Be Objective - need to be objective or impartial when reviewing an article e.g. don't fall into the scenario to review an article badly because the article author didn't cite the reviewer's research artworks.

5) Be Punctual - keep your deadline as promised else total cost of ownership for the blind review will be higher e.g. editor / coordinator have to chase you, article author lost the opportunity to publish in the journal that publish annually / half-yearly etc. (assess the time you need after due consideration & commit on the deadline mutually agreed).

Ideally, to qualify to review a paper, the potential reviewer should have considerable expertise in the field. S/he should have published or submitted papers in the field. I was in a seminar and an editor said that she uses ''her good reviewers'' often. We asked her to explain what a good reviewer is, and she said that a good reviewer is timely and thorough whenever s/he accepts to review a paper. She said that some researchers will accept to review a paper, but will never return the paper on time. These factors may determine who is selected to review an article.  

Good experience in research, good publication record, ethics, etc.