  We report a mean-field theoretical study of a triangular lattice magnet
NiGa$_2$S$_4$. Specifically, spiral mean-field theory is applied to a 17-band
$d$-$p$ model constructed from the maximally localized Wannier functions. Our
itinerant-model approach shows that the most stable spiral magnetic state has
an ordering vector near ${\bf Q}=(0.15,0.15,0)$, consistent with neutron
scattering experiments, when we assume the Ni-site Coulomb interaction is not
so strong ($U \approx 2$ eV). To map onto a classical Heisenberg spin model, we
estimate spin exchange interactions from the mean-field results, and find that
the nearest-neighbor exchange is ferromagnetic and the largest (larger than the
third nearest-neighbor exchange, in contrast to early studies). We also
calculate the dynamical spin correlation function $S({\bf q}, \omega)$, using
the same model within the random-phase approximation (RPA). Calculated $S({\bf
q}, \omega)$ has a spectral structure quite different from that of conventional
spin-wave excitations.
