How do I convert between eV and J

eV means electron volt - that is, it the amount of energy required for an electron to travel down (or up) a potential gradient with a potential difference of 1V. This energy, known as the work done, W, is given by W = q x V, where q is the charge and V is voltage. In our case the charge q = e = 1.602E-19 Coulombs and V = 1 Volt so the work done W = e x V = 1.602E-19 J. As a result 1eV = 1.602E-19 J.
As an example suppose you have calculated the energy, E, of a photon to be E = 3.97E-21 J. To convert this into eV first remember that 1eV = 1.602E-19 J. We can rewrite this as 1J = 1/1.602E-19 eV. Then simply replace J with 1/1.602E-19 eV to get
E = 3.97E-21 * (1/1.602E-19 eV) = 0.025 eV.