NOvA is a long-baseline neutrino oscillation experiment optimized for electron neutrino appearance in the NuMI beam, a muon neutrino source at Fermilab. It consists of two functionally identical, nearly fully-active liquid-scintillator tracking calorimeters. The near detector (ND) at Fermilab is used to study the neutrino beam spectrum and composition before oscillation, and measure background rate to the electron neutrino appearance search. The far detector, 810 km away in Northern Minnesota, observes the oscillated beam and is used to extract oscillation parameters from the data. NO?A's long baseline, combined with the ability of the NuMI beam to operate in the anti-neutrino mode, makes NO?A sensitive to the last unmeasured parameters in neutrino oscillations- mass hierarchy, CP violation and the octant of mixing angle theta23. This thesis presents the search for electron neutrino appearance in the first data collected by the NO?A detectors from October 2013 till May 2015. Studies of the NuMI neutrino data collected in the NO?A near detector are also presented, which show large discrepancies between the ND simulation and data. Muon-removed electron (MRE) events, constructed by replacing the muon in muon neutrino charged current interactions by a simulated electron, are used to correct the far detector electron neutrino appearance prediction for these discrepancies. In the analysis of the first data, a total of 6 electron neutrino candidate events are observed in the far detector on a background of 1, a 3.46 sigma excess, which is interpreted as strong evidence for electron neutrino appearance. The results are consistent with our expectation, based on constraints from other neutrino oscillation experiments. The result presented here differs from the officially published electron neutrino appearance result from the NO?A experiment where the systematic error is assumed to cover the MRE correction.