Let {v1,v2,v3} be a basis for vector space V. Prove that, if w is not in sp(v1,v2), then S = {v1,v2,w} is also a basis for V.

I know that in order for S = {v1,v2,w} to be a basis for V, the set S must span V and S must be linearly independent. However, I have no idea whatsoever on how to start this proof. Please help.