Thank you for this additional information - it certainly helps to clarify things! Your background research on the first law of thermodynamics is headed in the right direction. I suspect part of why you are having trouble finding background information is that the outcome of your project is related to the mass and temperature of what you add to the water, not the fact that your are adding mint. For example, you could get a similar result by adding an equal mass of candy bar that has been stored at the same temperature as your Mentos. That doesn't make your project any less interesting, but it does help to focus your background research.
The change in temperature of the water depends on the amount of energy transferred from the water to the Mentos, as well as something called the "heat capacity" or "specific heat" of the water. Have you learned about heat capacity in any of your classes? If not, then the link I included in my first reply is a good place to start learning about heat capacity. A technique called "calorimetry" (loosely, the measurement of heat transfer) is also relevant to what you are doing. These two web pages can get you started:http://dwb4.unl.edu/chem/chem869w/chem8 ... fo014.htmlhttp://www.physicsclassroom.com/class/t ... alorimetry
In brief, temperature is a measure of the average kinetic energy of the molecules in a material. When you add the Mentos to the water and they dissolve, the heat that is transferred from the water to the Mentos decreases the average
kinetic energy of the water + Mentos system, decreasing the temperature of the water.