It's been a while so I don't remember too well, but I'm pretty sure it was one of the most basic, Bb to C... My instructor has a thing with working gradually, little by little. Can't get a higher trill unless you get the lower ones first.
Then I started in grade seven I was being taught by a grade 10. I went to her one day to ask her what the tr on my music was, and she said it was a trill, from F to G. I asked how you trill from F to G and she said "Very easily?". Very helpful.
She soon realised I didn't know what a trill was and showed it to me. It was very easy.
f-g was my first. very easy because the two notes are pretty much the same embochure (almost) and so it's easy to get a nice sounding trill. plus i guess G is my strongest note
The first year i started playing, me and about 3 other flutes figured out how to do the F-G trill, and we would put it in EVERYWHERE! It annoyed our director so much. Every time there was an F, there was also a gauranteed trill from us =P