Internal energy is the energy contained within in a system. It is perceived in microscopic level when atom or molecule experience movement without external force field. i.e if u push a box to move it experience kinetic energy which is not taken as internal energy; but u can consider a littre of benzene contained in highland posses internal energy due to benzene molecular moments. The change in internal energy can be calculated by taking reference point eg. At 0K and 273K. The formula is in somebody's answer, no need to repeat!