Garter stitch is what you get when you knit every row so the term you want to use is "knit". For stockinette knit flat you knit a row then purl a row.
Now...you pick up your knitting with the stitches on the left needle and the working yarn on the right end by the needle point so you're ready to work. Look at the stitches while it's in your left hand so the work is facing you. Is the bumpy side facing you or the smooth side with the V's? The bumpy side is the purl side so you purl on that side. The V's are the knit side so you knit those stitches. Here's a sample of what the two stitches look like and what they look like close to the needle. Learning to read your knitting is an important skill.