You can register as a teacher as soon as you have a university degree or if you are a student that has successfully completed a university course.
Your profile is validated once you have completed your online application and uploaded your diploma or your full work experience or situation details. Usually you receive a profile validation email within two business days at most.
Yes it is free to register.
The ranking of each profile in the search results depends on a set criteria: mainly your star ratings and reviews, the speed with which you respond to requests, the frequency with which you log in, the number of works that you perform, etc.