If you do all purl stitches, then it will look just like when you did all knit stitches. If you want to be able to tell the difference between them, then you need to do 1 whole row of knit stitches and then a whole row of purl stitches. Alternate those 2 rows -- 1 row K, 1 row P, etc................
Fortunately, there only 2 basic stitches to learn - Knit and Purl. So once you've got those down pat, the rest is simpler!!!