By eating a varied and healthy diet and drinking enough, you can concentrate better and your memory will likely work better. Try to eat as many different (healthy) things as possible. Think about eating vegetables, fruits, nuts, and grains/bread. Also, try to drink about two liters per day. Be careful with soda and energy drinks: due to the large amount of sugars, they first provide an energy boost, but then your energy plummets, which can lead to a slump. The caffeine in coffee and energy drinks can help with taking a long test, but during studying, it can cause restlessness. Do not drink this either when you study in the evening: it will make you sleep less well, while you really need it.
Benton, D. (2011). Dehydration influences mood and cognition: a plausible hypothesis?. Nutrients, 3(5), 555-573
Edmonds, C. J., Crombie, R., Ballieux, H., Gardner, M. R., & Dawkins, L. (2013). Water consumption, not expectancies about water consumption, affects cognitive performance in adults. Appetite, 60, 148-153.
Fadda, R., Rapinett, G., Grathwohl, D., Parisi, M., Fanari, R., Calò, C. M., & Schmitt, J. (2012). Effects of drinking supplementary water at school on cognitive performance in children. Appetite, 59(3), 730-737.
Gómez-Pinilla, F. (2008). Brain foods: the effects of nutrients on brain function. Nature Reviews Neuroscience, 9(7), 568.
Psaltopoulou, T., Sergentanis, T. N., Panagiotakos, D. B., Sergentanis, I. N., Kosti, R., & Scarmeas, N. (2013). Mediterranean diet, stroke, cognitive impairment, and depression: a meta‐analysis. Annals of neurology, 74(4), 580-591.