Must I have a good level of English to join?
No, not necessarily, but you will need to be able to speak to and understand your teacher. Usually we find that students who have 1 or 2 years of English or who are of pre-intermediate level or above should benefit greatly from our service.
Note that we have a lot of teachers who speaks a second language. If you worry that your level of English might be too low for the live lessons, let know your academic advisor and he or she will match you with a native speaker that speaks your language.