Look up the specific heat capacity of your object. A list of common specific heat capacities is included in the resources section. For example, assume you want to know the amount of heat for magnesium. Its specific heat capacity is 1.020 J/g*degrees C.
Write out each of your variables. In the example, assume your change in temperature is three degrees, and your mass is 25 grams.
Multiply the three variables together. In the example, three degrees times 25 grams times 1.020 J/g*degrees C, equals an amount of heat of 76.5 J.