Your assignment topic is "Chocolate can make you happy. Discuss."
You need to select appropriate sources that includes information about the effect of chocolate on humans.
You may select:
The best place to start any research topic is with a dictionary that defines your keywords. This gives you a better understanding of your topic. From there, find an encyclopedia or text book that covers the history or origins of your topic, and may lead you to related topics. Then you can find journal articles which usually contain the latest research. Don't forget about digital resources like eBooks and online videos (just stick to the educational ones which may not be on YouTube).