I'm a third year student, but I'm taking first year French.
I went to The Tank today to buy my books, completely forgetting that first year books aren't sold in there.

I thought they were sold in Titles, but I didn't see anything there. I was told by someone at The Tank that they were sold at T13. Then I saw a poster outside The Tank that said all first year books are sold at The Tank.

So I'm very confused. Where can I buy first year French textbooks?

It depends on the field. Different fields of books are sold in different places.
http://titles.mcmaster.ca/text/location.htm
This gives the locations of where books for each field is sold during each time. I believe french would fall under humanities and social science (since there isn't anywhere else a language would possible fit), so until september 11th, they will be in T-29.

They will be sold in T29...but I'm not sure when. September 2nd and onwards for sure...it's just beforehand I don't know about.

French books are in T29 in front of Commons/Woodstock until Sept 11. Then I believe they'll be in the Tank.