You need to have an impressive portfolio that represents you artistically or in a fashionable/marketable sense.
There are plenty of freelance photographers out there, check the internet, your local guides etc. Do get reputable photographers though - they know how to capture you from a more artistic angles and bring up perspectives to best represent you.
Sometimes there are also scouts on the streets looking for other potentials. They are always on the look out for potential models.
If they spot you and see something in you, they'll give you the agency's name card and they'll want to see you and shoot you, examine you, criticize you, appraise you and see if you have the "it" factor or not.
You either have "it" or you don't.
If you choose to approach a particular agency, please do your research prior to rocking up there. Search the internet and you'll be able to find out where peeps like Naomi Campbell, Laetitia Costa, Christy Turlington etc go. These are the "Supermodels"...the forefront of modelling industry. It is true that your chances of obtaining jobs or more jobs (ie: advertisements, catwork, commercials etc) is via being represented by a very reputable and worldly-renounced modelling agency.
Each agency has its own conceptual style some go for the mixed look, rarer looks, orientals, - it's all about style and marketability.
Be aware that the modelling industry is HIGHLY competitive. It has its ugliness.
Also modelling for males are different as to the fundamentals behind it as supposed to the females.