The discovery of neutrino oscillations has proven that neutrinos have mass, and yet, for all the efforts to constrain the neutrino masses via direct and indirect methods, the neutrino mass scale remains undetermined. The KATRIN experiment aims at measuring the effective electron antineutrino mass with a 0.2 eV (90%C.L.) sensitivity by analysing the beta-decay spectrum of molecular tritium near its endpoint. This 70 m-long experiment consists of a windowless gaseous source, differential and cryogenic pumps for tritium retention, and a main spectrometer acting as a high-pass filter for the electrons collimated towards the 148-pixel silicon detector. This talk will focus on the commissioning of the KATRIN experiment, with an emphasis on its first tritium runs. The description of the beta-decay spectrum near the endpoint, the response function of the apparatus, and the statistical tools used for the interpretation of the observed integrated spectrum will be highlighted.