How to get study permit

[doctorA]

Hello,
I am currently in Canada as a visitor, I would like to know can I apply for the study permit from within Canada?
I have gotten admission in University, but as I am on visit visa and want to apply for the study permit from within Canada. Can I apply online? or have to apply by paper?
Does anyone else has experienced this same situation? What are the documents requirements?
I am very worried as in September my classes will start.

Thanks in advanc