Obtaining online information on the onset of great solar energetic particle (SEP) events from real-time data of the neutron monitor network (NMN) is considered and the corresponding algorithm and program are proposed. Determination of the particle energy spectrum outside the atmosphere at different moments of the flare is considered on the basis of coupling functions method. The spectra defined in diffusion and kinetic approaches are compared. Using this information, the time of the SEP ejection into solar wind, the energy spectrum of a SEP event in the source inside the solar corona, and the SEP diffusion coefficient in the interplanetary space during the flare can be estimated. In this work, the significant possibility of the expected SEP fluxes and the energy spectrum forecasting on the early part of the increasing SEP intensity (about 20-30 min after the onset) is considered. Available satellite data in real-time scale combined with real time-data from neutron monitors (NM) are used for extrapolation of this forecast to the region of very small energy particles. The method is checked on the SEP event of September 1989. It is important to note that the accuracy of the developed method sufficiently increases with the increasing dangerous level of the SEP event. The method is not CPU damaging and can run in real time, providing inexpensive means of SEP prediction.