The nucleophile is usually not part of the organic molecule, but it could be. It always has a lone pair. It is stable with extra positive charge, or can lose H+.
This means finding the leaving group. A leaving group has two important characteristics: It has a polarized C-LG bond, caused by electronegativity differences. The leaving group is stable after leaving. Find the C the leaving group is attached to: that's the point of attack (C1).
In this case, the nucleophile approaches from the back, while the leaving group leaves simultaneously. A new molecule is formed in 1 step with inversion of configuration at C1.
A typical SN2 reaction is performed on an unhindered substrate (1°, 2°) with a good LG, and is carried out by a strong nucleophile.
In this case, the leaving group leaves first, generating a carbocation. That leaves open the possibility of the nucleophile coming in from either side, forming a new molecule in 2 steps. If the starting material is chiral and optically active, this pathway invariably produces racemization.
A typical SN1 reaction is carried out on a substrate which will stabilize a carbocation at C1 (3° preferred, 2° possible). The nucleophile needn't be very strong, although the leaving group should be a good one.
