Here's a link
to the pattern.
If you want to knit this pullover in the round, you'll join the stitches to knit in the round. Cast on twice the number of stitches given for the back for your size, follow the directions for the Moss stitch in the round
and then change to knit every round (stockinette stitch). The yarn tail will mark the beginning of the round. When the knitting measures 17.5" (for the smallest size) you're at the armhole. You could bind off 2sts at the beginning of the round and then two sts halfway around for the armhole. For the smallest size that means bind off 2sts, knit 76sts (counting the one stitch left after the bind off), bind off 2sts and then knit 76sts and you should be at the end of the round. From here on, you have to knit back and forth on the back, finish it, then knit back and forth on the front (knit one row, purl one row for stockinette). You can continue with the circular needles, you'll just be knitting back and forth. You can put the sts for the front on another circular or on a stitch holder or on some waste yarn while you knit the back. The only other change you have to make is to account for the stitches you bound off when you come to the end and bind off for the shoulders.
Good luck with the sweater. First time is always an adventure.