How do I know if an enthalpy change should be positive or negative?

A negative enthalpy change represents an exothermic change where energy is released from the reaction, a positive enthalpy change represents an endothermic reaction where energy is taken in from the surroundings. When deciding if a change should be exothermic or endothermic, and calculating the enthalpy change, we are basically working out how many bonds we've broken and how many we've formed. Bond breaking requires energy, and forming bonds releases energy. This applies to more than just covalent bonds, when forces of attraction between molecules ae formed this also releases energy and vice versa. In working out the enthalpy change we are effectively just adding up energies gained and lost in the reaction. Born-haber cycles are a way of doing this, we add up the enthalpies of each change that is taking place in the reaction. For example, when working out the enthalpy of formation of NaCl, when the sodium and chloride ions come together to form NaCl, this is one component of the overall reaction known as the lattice enthalpy, which we add up with the other components (bond energies, ionisation energies etc) to determine the overall enthalpy of formation. Should the lattice enthalpy be positve or negative? Well, we are bringing together two oppositely charged ions, forming an ionic bond, this will release energy, it's an exothermic change, the lattice enthalpy must be negative. This same logic can be applied to all other thermodynamic changes.