Using size 8 needles, cast on 40 stitches with worsted weight yarn. (You can cast on any multiple of 4 stitches to produce a skinnier or wider scarf.) Follow a knit 2, purl 2 pattern for each row until your scarf reaches the desired length. Bind off.
I used about 2.5 skeins (about 400 yards) of Vanna's Choice Lion Brand yarn in taupe to make a scarf for my very tall (6'7") fiance. Two skeins (340 yards) or less should be enough for most people. Vanna's Choice is not an exciting yarn, but it's great for beginners. It's inexpensive, widely available, 100% acrylic (washable/dryable), and neither scratchy nor overly soft.
By the time you finish, you should have a good sense of knitting the purl stitches and purling the knit stitches, which is an instruction you'll see in more advanced patterns. Additionally, you'll be a pro at switching your yarn from front to back in order to change from purl to knit, and vice versa.