I am a beginner knitter. I am knitting a scarf - cast on 19 then knit purl…on the second row do I start with knit or purl?

It depends if you want ribbing or seed stitch. Look at this threadto learn how to tell a knit st from a purl stitch on the row you’re knitting. For ribbing you knit the knits and purl the purls; for seed st you purl the knits and knit the purls.

It depends on the pattern you are knitting. If you are making a rib stitch (even rows of knits and purls) then you will knit the knit stitches and purl the purl stitches. If the last stitch on your needle before you turn your work and start the next row is a purl, when you turn your work, you will see the back of the purl stitch which will be a knit stitch. So that row will start off with a knit stitch.

If you are making a seed stitch pattern (a checkerboard pattern of knits and purls) you would knit into the purl stitches and purl into the knit stitches. So if your last stitch on your row is a purl, when you turn your work and start your next row, your knit stitch is facing you but you would begin with a purl.

If you’re just beginning to knit, I’d use a rib stitch instead of seed. Seed is harder to figure out if you’re not very experienced, yet. Since you have an odd number of stitches (19), on the first row, you start with a knit stitch, and then purl1, knit1 to the end. On the 2nd you start with a purl stitch, and then knit1, purl1 to the end. Just keep alternation these 2 rows.