  We present the discovery of the Type II supernova SN 2023ixf in M101 and
follow-up photometric and spectroscopic observations, respectively, in the
first month and week of its evolution. Our discovery was made within a day of
estimated first light, and the following light curve is characterized by a
rapid rise ($\approx5$ days) to a luminous peak ($M_V\approx-18.2$ mag) and
plateau ($M_V\approx-17.6$ mag) extending to $30$ days with a fast decline rate
of $\approx0.03$ mag day$^{-1}$. During the rising phase, $U-V$ color shows
blueward evolution, followed by redward evolution in the plateau phase.
Prominent flash features of hydrogen, helium, carbon, and nitrogen dominate the
spectra up to $\approx5$ days after first light, with a transition to a higher
ionization state in the first $\approx2$ days. Both the $U-V$ color and flash
ionization states suggest a rise in the temperature, indicative of a delayed
shock breakout inside dense circumstellar material (CSM). From the timescales
of CSM interaction, we estimate its compact radial extent of
$\sim(3-7)\times10^{14}$ cm. We then construct numerical light-curve models
based on both continuous and eruptive mass-loss scenarios shortly before
explosion. For the continuous mass-loss scenario, we infer a range of mass-loss
history with $0.1-1.0\,M_\odot\,{\rm yr}^{-1}$ in the final $2-1$ yr before
explosion, with a potentially decreasing mass loss of $0.01-0.1\,M_\odot\,{\rm
yr}^{-1}$ in $\sim0.7-0.4$ yr toward the explosion. For the eruptive mass-loss
scenario, we favor eruptions releasing $0.3-1\,M_\odot$ of the envelope at
about a year before explosion, which result in CSM with mass and extent similar
to the continuous scenario. We discuss the implications of the available
multiwavelength constraints obtained thus far on the progenitor candidate and
SN 2023ixf to our variable CSM models.
