Garter stitch when done in rows means you knit every row OR purl every row. Garter stitch creates bumps on both sides of the work. At first glance they may appear to be purl stitch bumps, but if you pull the fabric apart slightly, you will see the 'V' like stitches in between which tell you there are knit stitches there.
Stockinette stitch done in rows means you alternate knit and purl rows. This create a knit side and a purl side of your work. Stockinette stitch is the stitch most often used to measure your gauge or tension. In fact, if a pattern does not say what to use to measure gauge, they assume Stockinette Stitch (abbreviated 'St st').
When working in the round, you can create garter stitch by alternating knit and purl rounds. Because you never turn your work when 'in the round', you are always working on the right side of the work.
To do Stockinette stitch in the round, you just 'knit, knit, knit'. Since you are always working on the right side, and in Stockinette stitch the right side is the knit side, all you have to do is knit. Reverse Stockinette stitch(abbreviated Rev St st) is created the same way as St st, but the right side is the purl side instead of the knit side.
learn how to identify what purl and knit stitches look like
watch videos of how to make a slip knot and long-tail cast on, how to bind off, how to add yarn, and identify knit stitches and purl stitches
watch videos the basic increase and decrease stitches
watch videos of english and continental styles of the knit and purl stitches
watch videos of knitting in the round, other ways to cast on, cabling techniques and colorwork