  We have begun a program to discover high-redshift supernovae ($z \approx$
0.25--0.5), and study them with follow-up photometry and spectroscopy. We
report here our first discovery, a supernova at $z = 0.458$. The photometry for
this supernova closely matches the lightcurve calculated for this redshift from
the template of well-observed nearby Type Ia supernovae. We discuss the
measurement of the deceleration parameter $q_0$ using such high-redshift
supernovae, and give the best fit value assuming this one supernova is a
normal, unextincted Type Ia. We describe the main sources of error in such a
measurement of $q_0$, and ways to reduce these errors.
