Really, it does not matter. Most patterns do not specify which to start with, and if they don't, then you can start with whichever one you want. However, some patterns will specify. For example, a...
If you are alternating each stitch (knit, purl, knit, purl) the pattern is called ribbing. If you alternate each row (row 1 knit, row 2 purl) it is called stockinette stitch.
You always make one to knit unless it says specifically to do it as a purl but make sure to make one the English way . After you pick up the bar between the two stitches knit it thru the back so you...