Answer:
You can become a model by having plenty of self confidence. Start off by shadowing real models (if allowed) and get a feel for what their job is really like. Then ask the models how they got started and they might even point you towards a direction.
You can get signed by emailing companies about your experiences with other models. Tell them how passionate you are about the job.
The best teen modeling agency just depends on where you live. Its different in each place.
I HOPE THIS HELPED <3