The magnetic structure of LaMnO3 and the antiferromagnetic resonance frequencies in it are investigated as functions of magnetic field H and temperature T. A previously developed model that includes the orbitally dependent superexchange and single-ion anisotropy is used. The temperature dependences described in the framework of the mean field approximation are in good agreement with experiment. The H - T phase diagrams are constructed for magnetic field directed along the principal crystallographic directions (a,b,c). The problem of the origin of the weak noncollinearity of the Mn3+ spins is solved. An optimal method of obtaining the expression for the magnetic free energy is proposed.