If you know which company you would like to work for but don't know if they have any vacancies, it is worth sending them your CV and asking if they have any openings. This is called a speculative application.
A speculative application is aptly named, a speculation is a gamble. You are gambling that they will have a suitable vacancy when you approach them. Speculative applications are not for those who are easily hurt by rejection as quite often companies that you approach won't have a vacancy. If this is the case, then at best you'll receive a brief letter of rejection, but they probably won't even reply to you, don't be surprised by this - after all, they didn't ask you to approach them.
When organising your job search you should firstly, apply to jobs you know exist, i.e. those that are being advertised.
Secondly check other possible vacancy sources i.e. recruitment agencies.
Then it is worth spending time on making speculative applications to companies of your choice.