First, try adding a beam of light by clicking the "Beam" button and then clicking again when you've moved the mouse to the point you'd like to place the beam. Add a mirror or lens by following the same procedure.
You can change the focal length of a lens or mirror by clicking on it to make it active. Then click-and-drag the focal point (each focal point is a white dot) to move its position. Moving it through the lens or mirror changes it from converging to diverging (or vice versa).
Investigate ray diagrams by replacing the beam with an object. Move the object around to see what happens to the image, and play with the focal point of the lens or mirror to see how that affects the position and height of the object.
You can use more than one lens or mirror. See if you can create a simple microscope, for instance, where the final image is virtual, and larger than the object.
This simulation is based on a physlet by Wolfgang Christian, part of the physlets page at Davidson College.