In knitting, "Cast On" is the term to describe the act of making stitches and placing them on a needle so one can knit them. The cast-on row becomes the bottom row. The base from which you'll knit.
It's important to remember that while knitting involves two needles, one--the needle in your left hand--is used solely to hold stitches. Nothing else! Before I started knitting, I wasn't aware of that.
So to begin, you have to put stitches on the left needle. How many? That depends on your pattern, which we'll talk about later. Right now, just shoot for ten to twenty.
There are numerous ways to cast on, but the easiest is the knitted cast on. I used this method for over a year. I could create a video to show you, but why when Lion Brand has a clear, professional video on their YouTube channel? You'll want to bookmark these videos in the event you need to refresh your memory.
Make a slip stitch, slide it on the needle in your left hand, and start casting on!
(Note, these videos are for right-handed knitters. Videos for left-handed, or Continental style, knitting are also available online.)
Practice this technique several minutes each day. Next up--the knit stitch.