Usually the first row is the RS so would be a knit. Even if you’re using a Longtail caston. Many instructions for it say that it creates your first knit row, but it’s pretty much hidden, so it’s considered the cast on not a row.
Are you going to put a border on it? If not, you may want to start with a few rows of garter or seed stitch to keep the ends from rolling. And several sts at each end of the row in garter or seed to keep them from curling under.
Hi! Whether or not you start with a knit or a purl for the first row depends on whether or not your afghan is reversible. Is there a definite front and back? Usually the odd rows are the wrong side (back) and the even rows are the right side (front). So when the pattern starts with row 1 starting with a St st, I would start with a purl.