Large-scale Monte Carlo simulation results for the two-orbital model for manganites, including Jahn-Teller lattice distortions, are presented here. At hole density x=1/4 and in the vicinity of the region of competition between the ferromagnetic metallic and spin-charge-orbital ordered insulating phases, the colossal magnetoresistance (CMR) phenomenon is observed with a magnetoresistance ratio ∼10,000%. Our main result is that this CMR transition is found to be of first order in some portions of the phase diagram, in agreement with early results from neutron scattering, specific heat, and magnetization, thus solving a notorious discrepancy between experiments and previous theoretical studies. The first order characteristics of the transition survive, and are actually enhanced, when weak quenched disorder is introduced.