While most of the agencies come directly to the candidate, you can also consider calling them. You need to identify agencies that have a job opportunity that is ideal for you. You can share your resume with them and go to pre-arrange meetings.
Alternatively, the recruitment agency will reach you by going through your online CV and when they would find that you are a perfect fit for the role.
In both the cases, the recruitment consultant will act like a middle-man between the company and you. They would share your CV with the company and would plan an interview. If everything results positive, then they will help you with interview preparation and offer you tips and pointers and direct you to seek the best results from the interview.