Generally whenever I see a tr I start it on the note that the tr is written over.
I always trill up (never down).
In classical and baroque, one trills -- unless it is written specifically otherwise -- starting on the upper note. If the composer meant otherwise, they will use a different ornament (there are whole catalogues of these things!). Further, the trill is meant to be in time. In this instance, start on the upper note (C#). My inclination would be to make it into six notes total -- C#,B,C#,B,A,B -- as that seems to fit the overall tempo and feel.