When a conductors carrying current is placed in a magnetic field, the field exerts a force on the conductor and vice versa. Fleming's left hand rule is a tool that you can use to help work out the relative directions of current, motion (force) and magnetic field. [Use diagram to show hand use. Thumb - motion, first finger - field, second finger - current].
Then demonstrate how to use the hand to help answer typical questions as I have found that students often struggle to actually make the correct shape with their own hands. Get them to use this technique to answer question: if current is flowing into the page and the field lines are going away from you which direction is the wire going to move? Answer: to the right.