Review http://www.physicsclassroom.com/class/light/U12l3b.cfm
Read http://www.physicsclassroom.com/class/light/U12l3c.cfm and do the two questions at the bottom.
Read http://www.physicsclassroom.com/class/light/U12l3d.cfm
Then, practice setting up the experiment, using the applet