How can I get the training data from a Landsat Image?

I need to do classification using the knnclassify command but I am confused with creating the training matrix. I have the following information.

nchannels=12; nx=949; ny=220; Data = lanread('flc1.lan',nchannels,nx,ny);

This creates a matrix of 949x220x12 matrix. How can I create a training matrix to use knnclassify. I must do this without any clustering. Looking forward for a reply.