Calculating enthalpy change
Enthalpy change is negative when the system releases energy, and it can be found from q = mcT or from formation enthalpies.
A worked example
Warming 250.0 g of water by 12.0 degrees C absorbs q = 250.0 x 4.19 x 12.0 = 12 570 J, about 12.6 kJ.
The mistake people actually make
For formation enthalpies, subtract reactants from products, and an element in its standard state has a formation enthalpy of zero. A calorimeter measures the surroundings, so water warming up means the reaction was exothermic.