You would have to submit an invitation letter written by your friend. See the following link on what to include in the invitation letter:

Letter of Invitation
http://www.cic.gc.ca/english/visit/letter.asp

You also need to provide proof that you will leave by a certain date, strong ties to home country (or presently US that you are in the middle of your studies) and that you have sufficient funds (either by yourself or your invitor) to support yourself while in Canada. The TRV application has a document checklist that you can use as guideline.

Application for a Temporary Resident Visa to Visit Canada
http://www.cic.gc.ca/english/information/applications/visa.asp

Document Checklist
http://www.cic.gc.ca/english/pdf/kits/forms/IMM5484E.PDF

Also check with the embassy/consulate where you'll be applying from for any additional requirements

Good luck!!