Cast on 38 st. (26 stitches with bulky yarn). For a skinny scarf, cast on 18 (14 stitches with bulky yarn).
If you're making a regular scarf with regular yarn, CO 38 sts; if it's thicker yarn, CO 26 sts.
If it's a skinny scarf with regular yarn CO 18; for thick yarn 14 sts.
Row 1: Work 2x2 ribbing across the row, beginning with K2 and ending with K2. Turn.
K2 sts, p2 sts - repeat all the way across, the last 2 sts will be k2.
Row 2: Continue ribbing by working stitches as they appear, beginning with P2 and ending with P2.
On the next row, P2, k2 and end with p2 on the last 2 sts. If the st looks like a knit on this side, knit it; if it looks like a purl, purl it.
Repeat the last two rows until scarf is desired length.
Keep doing row 1, then row2, row 1, row 2 until it's as long as you want it.
Bind off stitches in rib (if the stitch is a knit stitch, knit it; if it's a purl stitch purl it; pull stitches over one another as usual).
Weave in ends.
Binding off is really only pulling one stitch over the other. Otherwise you're just ribbing like you did on the other rows.
You may want to check the Video section - http://www.knittinghelp.com/videos/learn-to-knit - if you still don't understand how to do a part of that.