OK - first, take a breath! ahhhh....doesn't that feel better?
For garter stitch on circular needles, you will knit one row & purl one row, since you never turn the work.
When you count rows in garter, you count each knit AND purl row, like this:
Each ridge is going to be 2 rows. SO, knit & purl until you count 17 ridges.
Make sure to place a stitch marker after your join so you know when to change to another type of stitch.
Have fun! That Booga bag is AWESOME!