  This is the first paper in a series dedicated to the study of the
emission-line and continuum properties of narrow line Seyfert 1 galaxies
(NLS1s). We carried out a systematic search for NLS1s from objects assigned as
``QSOs'' or ``galaxies'' in the spectroscopic sample of the Sloan Digital Sky
Survey Data Release 3 (SDSS DR3) by a careful modeling of their emission lines
and continua. The result is a uniform sample comprising $\sim 2~000$ NLS1. This
sample dramatically increases the number of known NLS1 by a factor of $\sim 10$
over previous compilations. This paper presents the parameters of the prominent
emission lines and continua, which were measured accurately with typical
uncertainties $<10%$. Taking advantage of such an unprecedented large and
uniform sample with accurately measured spectral parameters, we carried out
various statistical analysis, some of which were only possible for the first
time (abbreviated).
