A method based on static light scattering by fractal aggregates is introduced to extract structural information. in this study, we determine the scattered intensity by a fractal aggregate calculating the Structure and the Form factors noted, respectively, S(q) and F(q). We use the approximation of the mean field Mie scattering by fractal aggregates (R, Botet, P. Rannou, and M. Cabane, appl. opt. 36, 8791, 1997). This approximation is validated by a comparison of the scattering and extinction cross sections values calculated using, on the one hand, Mie theory with a mean optical index (ii) and, on the other hand, the mean field approximation, Scattering and extinction cross sections values differ by about 5%. We show that the mean environment of primary scatterers characterized by the optical index n(s) must be taken into account to interpret accurately the scattering pattern from fractal aggregates. Numerical simulations were done to evaluate the influence of the fractal dimension values (D-f > 2) and of the radius of gyration or the number of primary particles within the aggregates (N = 50 to 250) on the scatterers' mean optical contrast (n(s)/(n) over bar). This last parameter plays a major role in determining the Form factor F(q) which corresponds to the primary particles' scattering, In associating the mean optical index ((n) over bar) to structural characteristics, this work provides a theoretical framework to be used to provide additional structural information from the scattering pattern of a fractal aggregate (Cf. Part II).