@Szahra
First of all, there is a big difference between letter of acceptance and letter for explanation! I hope you understand this.
You can apply for study permit AFTER you got a letter of acceptance from a university in Canada. There are two ways to apply for study permit; online (by internet) and offline (paper based). For the latter you do not need to upload anything. I recommend you to apply by paper since you are not experienced with the procedure.
There are 10 vfs offices in India, you can apply in anyone. refer to this link for more information:
http://www.vfsglobal.ca/Canada/India/index.html.