Once you have decided to take modelling on a professional level and have your modelling portfolio you can market yourself straight away.
Start by selecting 4/5 best images form your portfolio. you must include a head and shoulder, a full length and a strong pose.
Select the agencies that are matching your criteria and start by attaching your images to an email along with all your statistics.
The agencies will contact you within 2-4 weeks, as your images will go through a selection process, which can take time. most won't reply if you are unsuccessful.
Once you are selected, you will be called in for an interview to sign a contract.
Do not stop there! many commercial agencies are not exclusive, this means that you can be signed to many different agencies at once and maximize your earnings and have the luxury to pick and choose which castings and shoots you would like to model for.