I know that depending on the era you can either start on the note or note above in some ornamentation such as a turn or mordent.
For Abrsm grade 8 theory do you start on the note or the note above so for example for a turn on a C would it be?
C D C B C
Or D C B C
Baroque music was the only era were you would start on the upper-auxiliary note. All the other eras you should start on the principal note. The turn must fit in the note values that are given. A slow piece may have more notes in a trill for instance and a faster trill less.
As is the way with these things you have certain amount of artistic licence in the playing of ornaments, as long as it fits and it leads neatly to the next note, you should be ok.
Ornaments in general begin on the written note. A turn written above C would be played C-D-C-B-C; a reverse turn would be C-B-C-D-C.
Most ornaments have a shape that sort of describes how you play it - the turn curls up, then down to its lowest point, then back to a point horizontal with its start. It's going to sound like the shape: up, back, down, back.
