This paper presents a rigorous solution to the scattering of a monochromatic plane wave by an arbitrary configuration of wavelength-sized small particles that can be of different shape, structure, size, and composition. A T-matrix formulation is developed for the calculation of optical cross sections and the asymmetry parameter of such an ensemble of scatterers in both fixed and random orientations. The solution is based on the T matrix T(jl), that is, the inverse of the coefficient matrix of boundary condition equations. A linear system containing T(jl) is derived to efficiently solve the T matrix, which is required in the practical implementation of the solution.