If you are doing alternating rows of knit and purl you are doing stockinette stitch.
You already know you hold needle with the stitches in your left hand so look at the stitches that are facing you. Are they bumpy or smooth? If they are smooth V looking stitches that is the knit side so you would knit. The bumpy side is the purl size so you would purl.
Learn what the stitches look like- here's a sample.