Tropism is a property plants exhibit as they try to maximize their ability to capture sunlight. Tropism literally means "a turning", meaning the plant will turn itself so as to catch the maximum amount of sunlight available. It does this because it needs the sunlight to produce food for itself, glucose, through a process called photosynthesis.
Get a plant in a pot, put the plant on a table by a window. Orient the plant so the leaves are facing toward the interior of the room, where it will naturally be darker. Make sure the curtains to the window are open, allowing the sunlight each day to flood into the room. Measure the amount of "turning" the plant does each day; this can be done by taking pictures over a week, then comparing the orientation of the plant in the pictures over the progression of time. You will observe the plant slowly but surely turning towards the sunlight, until the leaves are catching the maximum amount of sunlight they can from the position they are in.
Don't forget to water the plant---we are measuring the plant's ability to turn, and to do that, all the other factors need to be equal, not changing. Also, keep the temperature in the room about the same each day you are conducting the experiment.