My daughter did a bit of modelling when she was younger. To start with she had to have a proper "porfolio" done by a professional photographer and then she had to send it to all the modelling agencies and wait! Luckily she did get a few bits and pieces, altho she was never a "Kate Moss", but she did it for a couple of years and it gave her some pocket money and some "street cred"
I dont think you can simply phone up agencies and ask to become one. Certainly not with the reputable ones - as for unreputable ones... well looks dont really matter to them, they're more interested in what you will do, not what you look like!
So google some agencies and find out what they require in terms of portfoios, pictures, cv etc and take it from there.