  Chemical compositions are determined based on high-resolution spectroscopy
for 137 candidate extremely metal-poor (EMP) stars selected from the Sloan
Digital Sky Survey (SDSS) and its first stellar extension, the Sloan Extension
for Galactic Understanding and Exploration (SEGUE). High-resolution spectra
with moderate signal-to-noise (S/N) ratios were obtained with the High
Dispersion Spectrograph of the Subaru Telescope. Most of the sample
(approximately 80%) are main-sequence turn-off stars, including dwarfs and
subgiants. Four cool main-sequence stars, the most metal-deficient such stars
known, are included in the remaining sample. Good agreement is found between
effective temperatures estimated by the SEGUE stellar parameter pipeline, based
on the SDSS/SEGUE medium-resolution spectra, and those estimated from the
broadband $(V-K)_0$ and $(g-r)_0$ colors. Our abundance measurements reveal
that 70 stars in our sample have [Fe/H] $ < -3$, adding a significant number of
EMP stars to the currently known sample. Our analyses determine the abundances
of eight elements (C, Na, Mg, Ca, Ti, Cr, Sr, and Ba) in addition to Fe. The
fraction of carbon-enhanced metal-poor stars ([C/Fe]$> +0.7$) among the 25
giants in our sample is as high as 36%, while only a lower limit on the
fraction (9%) is estimated for turn-off stars. This paper is the first of a
series of papers based on these observational results. The following papers in
this series will discuss the higher-resolution and higher-S/N observations of a
subset of this sample, the metallicity distribution function, binarity, and
correlations between the chemical composition and kinematics of extremely
metal-poor stars.
