If you are asked this question in an interview it can be rather awkward knowing how to answer it. You obviously need to be very positive about yourself but also not sound big headed. You need to say things that will show you are the right person for the job but hopefully stand out from other people.
You may already know what your strengths are, but if you have never given it any thought, then you will need to before the interview. Consider your strengths in light of the skills required for the job so make sure that you most highlight those that the job requires.
Consider things like time management, organisational skills, reaching deadlines, customer service skills, communications skills and things like that.
It is possible that you could be asked which you feel is the greatest strength and how this helps you to perform well in your job. You will again need to focus this on what is required for the role and relate that in to a strength.
It is wise to prepare for a question about strengths by thinking of a real example of when your strength has helped you in your job. You may need to make a bit more out of a situation to make it sound grander than it is, but embellishing is okay as long as it still seems feasible and you are not lying.
Make sure that you do not just list the strengths required in the job description, in the order that they are written as it will look very obviously like you are just trying to be the person they want. Although you should include references to those strengths, you should also include other strengths that you have as well, which could be useful to the role, but perhaps not in such a direct way. You may even be able to be so convincing that you can make them believe certain strength would be critical to the role even if they are not listed as required.
Knowing how many strengths to list can be difficult. You want to give the question a detailed answer but not talk on for two long. Probably 3-4 strengths would be enough to demonstrate something about yourself without the risk of being boring or showing off too much. It may be wise to have more than that available to use though if you feel you need to talk for longer or if they ask you if you have any more.