The 'wrong side' of your knitted work is the side that will not show, so for instance the inside of a sweater is the wrong side. If you are following a pattern, it will say for example Row 1 (RS), so then you know that the odd numbered rows (1, 3, 5, 7, etc.) will be right side rows, which means that they will be on the side that will show when the item is worn or used. So that automatically means that the even numbered rows in this example would be wrong side rows.
Stockinette stitch is knit one row, purl one row, knit one row, purl one row, etc. The purl rows in stockinette stitch are the wrong side rows.
Garter stitch is knitting every stitch of every row. This produces two sides that look identical. So in the case of garter stitch, right side and wrong side don't really matter.
Hope this helps.