It is unlikely that any site will charge the teacher, as it is the teacher they need to teach the students. It is more likely they will charge the student, and then pay you, deducting a commission of say 10-20% commission. With some you set the price, and receive payment from the student using say paypal. I guess the site gets revenue from advertising.
A tip, do not accept agents who dictate your fee, as what you charge is your business. There are a lot of agents, try enrolling with as many as possible, search online english also try tutors classifieds, and skype tutors teachers
Best way is to advertise yourself, your own website, blogs, facebook, google. Also get some leaflets out in your area, advertising your services and your website.