Context. The most metal-poor stars are the relics of the early chemical evolution of the Galaxy. Their chemical composition is an important tool to constrain the nucleosynthesis in the first generation of stars. The aim is to observe a sample of extremely metal-poor star (EMP stars) candidates selected from the Sloan Digital Sky Survey Data Release 12 (SDSS DR12) and determine their chemical composition. Aims: We obtain medium resolution spectra of a sample of six stars using the X-shooter spectrograph at the Very Large Telescope (VLT) and we used ATLAS models to compute the abundances. Methods: Five stars of the sample have a metallicity [Fe/H] between -2.5 dex and -3.2 dex. We confirm the recent discovery of SDSS J002314.00+030758.0 as a star with an extremely low [Fe/H] ratio. Assuming the α-enhancement [Ca/Fe] = +0.4 dex, we obtain [Fe/H] = -6.1 dex. Results: We could also determine its magnesium abundance and found that this star exhibits a very high ratio [Mg/Fe]≤ +3.60 dex assuming [Fe/H] = -6.13 dex. We determined the carbon abundance and found A(C) = 6.4 dex. From this carbon abundance, this stars belongs to the lower band of the A(C)-[Fe/H] diagram. Based on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere under ESO programme ID 099.D-0576(A).