Don't worry about the cast on being a row or not. Some people count it as one, some people don't. If it's confusing you more I'd suggest just counting it as a cast on and no more. So the first row you actually knit will be your row 1. Knit across the 50 stitches and somehow mark that row so you know that it is your right side. A safety pin works great. Don't just mark it on the edge though, put the safety pin somewhere in the row so that it actually stays on that side of your work. The only reason that this is necessary is because with garter stitch, both sides look the same and your pattern is having you specify one side as the RS, right from the start.
So now, you will keep doing garter stitch, knitting every stitch of every row, until you reach whatever length specified in your pattern. You will end on a WS row, which means that the last row you knit will be the side that does NOT have the safety pin attached. When you turn your work around following that row, you will have the RS of your work facing you and you'll be ready to proceed with stockinette stitch. Stockinette stitch is knit the right side rows, and purl the wrong side rows. So since you have the RS of your work facing you, you will begin with a knit row and from there will alternate purl rows and knit rows until you reach the proper length, ending working a WS row which is a purl row.