Knitted items are built out of stitches, in the same way that a brick wall is made out of bricks: by stacking them up in rows. However, a knitted "wall" is incredibly simple: each brick, or stitch, is stacked directly on top of the previous one.
You must start your scarf by laying a foundation of stitches, which is called "casting on." Here's the easiest way to cast on - it's called the Backwards-Loop Cast-On.
Grab your yarn and one needle, and practice casting on along with the video.
When you've got the hang of it, begin your scarf by casting on 12 stitches (this includes the slipknot that you started with).
In order to get you used to the language of knitting instructions, here's how you would abbreviate that: CO 12 sts (the word "stitch" is abbreviated "st"). Pretty simple, no?