In reviewing papers for journals or conferences, what should one do if one is not fully knowledgeable in the specific area the paper addresses? This is a question from a newbie doing reviews for journals/conference papers.

What should one do if they are among the panel of reviewers for papers for a specific journal / conference, and you get a paper whose subject material you are not an expert in, or are not fully aware of, all the details around which the paper focuses on?

Is it the norm to state that you are not versed well enough in the particular area, thus you cannot make a qualified opinion, or should you just do as best as you can to review what you understand of the paper, and hope that the other reviewers will do a better job, compensating for your weakness? Are there any other options?
First off, "fully" is a very relative word, somewhere between expert and novice which makes a specific answer difficult at best. Editors try to identify persons they believe can provide a good constructive review of the manuscript in question. If that is the case, you are viewed as having that expertise. Note that it is not uncommon that an editor appoints experts with different specialities to cover different aspects of a manuscript. It is the responsibility of an editor to select reviewers with care to make sure a manuscript is scrutinized fairly and by peer "experts".

Now this system is not fool-proof and it is therefore the responsibility of a reviewer to decline to review if they think they are not in a position to take on such a review. There are of course many other reasons to decline but that is a different story. So in your case, you need to figure out based on the information you have received, if you have the background to provide input on all or significant parts of the manuscript in the request. When taking on the first reviews in a career, you may ask your advisor or peers about the task but remember, the fact that you have been asked is not necessarily something that is open information (in for example double-blind reviews where anonymity is requested).

So, think about how you can contribute. If you do not see that you can provide input, decline. Otherwise, take on the review. Reviewing is an integral part of academia and getting started is necessary at some point. It can also be quite rewarding since you can gain insights ito new science as well as ways to (or not to) disseminate science.