First think structure of aniline...Benzene ring with an NH2 substituent.
Then, think about resonance, particularly focusing on the nitrogen molecule.
We know the N molecule has 3 sigma bonds, one to C, and two to H, as well as one lone pair of electrons.
Therefore, we know that since the nitrogen has a p orbital in the same plane as the carbons on the benzene ring, it's lone pair of electrons will become delocalized and will participate in resonance.
When drawing res. structures, the N will LOSE electrons and become positively charged and form a C=N bond, and the adjacent carbon will become negatively charged with a lone pair.
This is important, as we now know that the -NH2 substituent is an electron DONOR, and thus it's charge on the sigma complex will be PARTIALLY POSITIVE, and the charges of the carbons at the ORTHO and PARA positions of the ring will be PARTIALLY NEGATIVE, and the meta positions will be partially positive.
Given that the amino is electron donating with a + charge, we can safely say that it is NOT a nucleophile, but rather an electrophile
