Do Trills Go up or Down?

Trills provide the flutes and other woodwinds the opportunity to add brilliance and excitement to a score. Forgive me for stating the obvious, but a trill is a rapid alternation between adjacent pitches, up either a half step or whole step from the notated pitch (never down).

How do you know when to trill up or down?

There are lots of rules that varied over music history. The shortest rule would be that in the Baroque and early Classical period trills start on the upper note (above the principal note = "trill down", I guess). And that later, there is more tendency to start on the principal note and "trill up."

Which way do you trill?

The usual way of executing a trill, known as a diatonic trill, is to rapidly alternate between the written note and the one directly above it in the given scale (unless the trill symbol is modified by an accidental, understood to apply to the added note above; this is a chromatic trill).