Résumé : The extremely metal-poor stars (EMP) hold in their atmospheres the fossil record of the chemical composition of the early phases of the Galactic evolution. The chemical analysis of such objects provides important constraints on these early phases of the Universe. The chemical composition of EMP stars can give insights on the star-formation of low-mass stars from a metal-poor gas and on the masses of the Pop.III first-generation massive stars. EMP stars are very rare objects ; to dig them out large amounts of data have to be considered. I wrote an automatic procedure to analyse objects with colours of Turn-Off stars in the Sloan Digital Sky Survey (SDSS). I selected good EMP candiadates to observe at higher resolution, with X-Shooter@VLT and UVES@VLT. The candidates have been confirmed to be metal-poor stars. I dedicated also some effort to understand the carbon enhanced EMP stars that are quite common for stars at very low [Fe/H]. I present here the procedure to depict the EMP candidates, the analysis, and the implication in star formation.