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