Generally the pattern will specify which is which, as in: Row 1 (RS), or it may say Row 2 (RS). The right side is to be worn facing out, so in stockinette stitch, the knit rows are the RS. In garter stitch it may not matter; you choose which is the right side if there is any shaping to be done for armholes or the neck. If you have a specific pattern, take a close look at it and see if it says which is RS and which is WS.
When a pattern says end on WS, does that mean that the WRONG SIDE will be on the left needle for the next row or does it mean that the WRONG SIDE is the last row completed (so the RIGHT SIDE would be facing you on the left needle when starting the next row)?