Context : The most metal-poor stars in the Milky Way witnessed the early phases of formation of the Galaxy , and have chemical compositions that are close to the pristine mixture from Big Bang nucleosynthesis , polluted by one or few supernovae . Aims : Only two dozen stars with ( [ Fe/H ] < -4 ) are known , and they show a wide range of abundance patterns . It is therefore important to enlarge this sample . We present the first results of an effort to identify new extremely metal-poor stars in the Milky Way halo . Methods : Our targets have been selected from low-resolution spectra obtained as part of the Sloan Digital Sky Survey , and followed-up with medium resolution spectroscopy on the 4.2 m William Herschel Telescope and , in a few cases , at high resolution on the the 9.2 m Hobby-Eberly Telescope . Stellar parameters and the abundances of magnesium , calcium , iron , and strontium have been inferred from the spectra using classical model atmospheres . We have also derived carbon abundances from the G band . Results : We find consistency between the metallicities estimated from SDSS and those from new data at the level of 0.3 dex . The analysis of medium resolution data obtained with ISIS on the WHT allow us to refine the metallicities and in some cases measure other elemental abundances . Our sample contains 11 new metal-poor stars with \left [ { Fe / H } \right ] < -3.0 , one of them with an estimated metallicity of \left [ { Fe / H } \right ] \sim - 4.0 . We also discuss metallicity discrepancies of some stars in common with previous works in the literature . Only one of these stars is found to be C-enhanced at about [ C/Fe ] \sim + 1 , whereas the other metal-poor stars show C abundances at the level of [ C/Fe ] \sim + 0.45 . Conclusions :