The chain stitch is abbreviated ch -- as in "ch 3" for chain 3 or "skip 1 ch" as an instruction to skip the next chain st.
The first step is to make a slip knot on the hook (the same way one would make a slip knot for knitting). Here is a video on making a chain stitch from Nexstitch. It doesn't start with a slip knot, however. It has an ingenious way to start without a knot.
In words, here is how to hold the yarn and start to chain (quoting from the booklet from the American Thread Company -- since it describes how to wrap the yarn around the hand better than I can.) --- The quotation starts after instructions on how to make a slip knot and assumes that the thread is already hanging from the hook.
"Hold the hook in right hand as you would a pencil, bringing the middle finger forward resting it about midway between the broad bar and top of hook. With the thread in back of hand, place thread between fourth and little finger, across palm side of fingers and over fore-finger, if more comfortable, wind thread over finger once. Do not hold thread too tightly. Hold the hook in left hand, insert hook in loop, pick up the main length of thread on hook (this is termed 'thread over' ...) and pull through loop. Repeat this ch for required length. On this foundation chain may be worked practically any stitch desired."
I hold yarn for crocheting the same way I hold yarn for knitting. (I'm a Continental style knitter.) I hold a crochet hook differently than I do a knitting needle. For crocheting, I have my middle finger farthest toward the end of the hook and hold the hook almost like a pencil. For knitting, I have my index finger farthest toward the end of the needle and hold the needle as a knife. The fingers in the left hand are used to pull the thread downward a bit to make it easier to pull the yarn through. Note also that the yarn starts out behind the hook.
I've been told that I crochet like a knitter and knit like a crocheter.The main reason people say that I crochet like a knitter is that I use the index finger on my left hand to wrap the yarn around the hook instead of turning the hook in order to catch the yarn.
The chain stitch is a multi-purpose stitch. It serves the same purpose as the cast on in knitting in that it can and usually is used as a foundation row. It also serves the same purpose as the yarn over in knitting in that it's used to make crocheting more lacy. It's used to make picots. It's also used as a substitute for other stitches at the beginning of a row. Instead of making a single crochet at the beginning of a row, one might be asked to chain 1 (or 2). Instead of making a double crochet at the beginning of a row, one might be asked to chain 2 (or 3). (Some people use the lower number -- because a ch 1 is the same length as a single crochet is in height. Some people use the higher number.) I also use chain stitches when I'm going to put crocheting down for a while. I chain a few stitches loosely so that if it unravels, I won't lose any of my work. Then before beginning again, I undo the chain.
Double or triple crochet stitches are often substituted for chain stitches at the end of a row or round.