We develop a nonequilibrium mode-coupling theory for uniformly sheared systems starting from microscopic, thermostatted SLLOD equations of motion. Our theory aims at describing stationary-state properties including rheological ones of sheared systems, and this is accomplished via two steps. Firstly, a set of self-consistent equations is formulated based on the projection-operator formalism and on the mode-coupling approach for the transient density correlators which measure the correlations between the density fluctuations in the initial equilibrium state and the ones at later times after the shearing force is turned on. The transient time-correlation function formalism is then used which, combined with the mode-coupling approximation, expresses stationary-state properties in terms of the transient density correlators. A detailed comparison of our theory is also presented with the related mode-coupling theory which is based on the Smoluchowski equation for Brownian particles under stationary shearing.