The semileptonic decay form factors of the double heavy B_c meson provide a unique opportunity to study the strong interactions between two heavy quarks. A fully relativistic model, with effective non-local quark-meson interactions, is used to compute semileptonic decay form factors, for both the B_c and a wide range of other heavy-light mesons. Using these form factors predictions for decay rates and branching ratios are obtained. The results are compared with other theoretical approaches and, where available, to experimental results. In addition, the radiative decay of B*_c is discussed.