A recruitment agency will help you find a job that is well suited to your needs – as long as it is well suited to theirs as well. You will find that you get the right kinds of opportunities when you work with the right recruiters, who are actively recruiting in the fields where you would like to work. If you are interested in a certain sector, or you work in a certain type of job (marketing, HR, teaching etc) then it always pays off to find a specialist recruiter.
First, make contact with the agency to see if they might be right for you. This is always better than sending in a CV without a proper introduction. It will make you more memorable and will give the right first impression. You should then be assigned a recruiter, who will work with you and pass on details of jobs that are relevant to your skill set.