If you are told to find the basis for a span of a set of vec

If you are told to find the basis for a span of a set of vectors A, would you use the basis of Null(A) or Col(A)

if you have a set of mxn vectors, is the basis for the method of the base Null (A), also this method is used if deseaa to determine an orthogonal set of basis vectors.

As requested by the base find a set of vectors, is determined using the base Col (A).