Agencies usually have an "open casting' type of day one day per week where you can walk in. If you are under 18, you may need a parent/guardian with you. They will take measurements, ask questions about you, what you are interested in, etc. and likely take some basic pictures with a photographer on site. Then they will let you know if they are interested in signing you either then and there, or a couple days later. Keep in mind, this is a process, if they sign you (and by "sign" it's an actual written contract you should read carefully), they will have you shoot with different photographers and THIS is how you build your portfolio. After a couple of shoots, they will start posting more of those pictures to their website, and if clients like what they see, they will ask the agency to contact you for an actual paid job shoot. Also keep in mind if you are under 18, you will have to get a work permit to do any of this every 6 months. My daughter had some great shoots and made a nice bit of cash before starting college, but she did not get picked up for an actual client job until about 8 months after signing and doing several photo shoots with different photographers to build her portfolio. So this takes time! Also, many of the shoots were during school hours and she had to turn a few down due to finals, etc. at school. Clients and photographers want you when they want you, so if that is on a Tuesday at 11am, it means missing school or saying no. Don't be afraid to say no... a good agency actually wants to promote you as a good student as well, so they will understand and they will still contact you for more opportunities.
One more thing to add - don't be afraid to mention it on applications or resumes. My daughter (at the age of 16-17) had to walk into a room of strangers, act professionally at all times and make connections with different artists, photographers, etc. You are always representing your agency and yourself, so these are important skills to have and fine-tune. Good Luck!