|MadSci Network: Neuroscience|
The best way to familiarize your self with the scientific literature on a topic is to head to your local public university library. There you can do a pubmed search and have online access to most of the journals. A good review article in one of the major psychology journals should give you the state of the art. You can also talk to professors studying memory so see if they have a project you can help with.
Subject selection is an important part of your experimental design. The subjects you select will determine how well you can generalize your results and how likely you are to get statistically significant results. Random selection from the general population will give the most generalizability, but you will need 100's of subjects. For a science fair project I would select high school students in the same grade, and possibly even the same gender. Aim for 20-30 subjects. This will let you generalize the results to students at your school.
Learning ability is generally described as different forms of memory. Look for an introductory text book, or talk with a local graduate student about what experimental paradigms there are for studying different forms of memory.
Try the links in the MadSci Library for more information on Neuroscience.