Ab initio investigation of structural, electronic and vibrational properties of nitrogen-doped fullerenes (C60-nNn, for n = 1-12) has been performed using numerical atomic orbital density functional theory. We have obtained the ground-state structures for C60-nNn for n = 1-12, which show higher stability with a single nitrogen in a pentagon and two nonadjacent nitrogen atoms in a hexagon. Nitrogen doping leads to structural deformation, with the diameter showing variation from (7.14 - 0.24) to (7.14 + 0.10) angstrom. The average diameter of C60-nNn shows a small decrease for n >= 5, with a minimum value of 7.06 angstrom for n = 12. The change in the average diameter signifies the volume contraction, which is also maximum for C48N12. The binding energy per atom is found to decrease as a function of the number of N atoms. The HOMO-LUMO gap is found to decrease with an increase in substitutional nitrogen atoms; however, no systematic pattern could be observed. The Mulliken charge analysis performed on all optimized geometries shows a charge transfer of -0.3 to -0.45 (or 0.3-0.45 electrons) from nitrogen to carbon atoms, resulting in nitrogen atoms behaving as electrophilic sites. The harmonic vibrational frequency analysis shows the absence of any imaginary mode. The vibrational frequencies are found to decrease with an increase in the number of nitrogen atoms in C60-nNn. The results obtained are consistent with available theoretical and experimental results.