Assuming you want smoothing of the lighting, you calculate a vertex normal by averaging the normals of all faces that share that vertex.
Calculating face normals - here's a diagram to help you follow the description:
First calculate vectors from points v0->v1 and v0->v2 - this can be done by subtracting v0 from v1 and v2 in turn giving 2 new vectors vec0 & vec1. You then take the cross-product of these vectors, which results in a new vector FaceVec which should then be normalised (ie, calculate the length of the vector using pythagoras, then divide each part of the vector by this amount) - this is your face normal.
[EDIT]While I'm at it, I'll point you in the direction of some example code -
http://www.gamedev.net/reference/articles/article1682.asp. Ignore the fact that most of the article is about OpenGL - just look for the 3 function for getFaceNormal, crossProduct and normalize.