You should check out the following 2 links (they are 2 different chapters of our "How to do a science fair project" help guide):
http://www.sciencebuddies.org/mentoring ... blio.shtml
http://www.sciencebuddies.org/mentoring ... _lit.shtml
Given that your experiment consists of plant and sunlight, I would envision some background research on photosynthesis, cellular respiration in plant cells (optional), and how exactly do most plants produce their own food using air, water, and sunlight. They are autotrophs after all, and looking at the process of photosynthesis in detail can reveal quite a lot about plants.
Hope this helps!