First off being a Nigerian citizen, you are not eligible for an ETA to Canada unless you are a US Permanent Resident, which it would appear you are not.
For a visitors visa as a student, it is pretty standard that you get a letter from your school indicating you're enrolled and that the duration of visit will not affect your studies.
If you're going as a tourist as you indicate, you don't need an invitation letter from a resident of Canada. You will need hotel bookings, proof of having booked a tour with a tour company (if that is your mode of tourism), etc
Finally as to whether it is better to apply from Turkey or Nigeria, it shouldn't make a difference. As a student in Turkey you are allowed to apply from Turkey per the Canadian Government website. You can apply online or using paper. See below:
If you are applying on paper we recommend that you apply for a visa
from your home country. However, if you are legally admitted to the
country you are currently in, you can apply at your nearest visa
application centre (VAC). The VAC can help make sure your
application is complete and that all proper documentation is included.
You must state clearly on your application why you have not applied in your normal country of residence.
Proof of Financial Resource of Supporter
Proof of Means of Financial Support
Purpose of Travel
Family Information (IMM5645)