Ok lets try and explain this legibly!
you have for example 20 stitches on your needle, the stitches go like this:
Black (B), White (W), B, W, B, W and so on
Gives you 10 pairs of 2 stitches, one B and one W.
Black is the front so will be knitted, white is the back so will be purled. Start with both strands in the back of your work, use the black strand only to knit the black stitch, bring both strands to the front of your work, using the white strand only purl the white stitch. This is the basic method and will give you a fabric of stocking stitch black on one side, and white on the other.
If you want to change colour, you would continue to move the strands forward and back together but use the opposite colour, so using the example it would be B W, B W, W B, B W...this would mean the the third stitch (remembering that each stitch has a pair) would now be the opposite colour than before. This also applies to knit and purl stitches so if you think of B W, B W, as K P, K P, instead you would then swap the K for the P.
Hopefully this makes sense