The chain stitch provides a base from which to begin your crochet pattern.
1. Yarn is attached to crochet hook with a slip knot. Pass the hook under the yarn so that hook catches the yarn as it lies across the top of hook; in crochet terminology - "yarn over hook" (yo).
2. Draw the yarn through the loop on hook to complete one chain stitch. (The first chain stitch is drawn through the loop made by the slip knot; the slip knot slips off hook as the stitch is completed, and another loop is automatically formed on hook. The next chain stitch is pulled through this loop, etc.)
3. To continue chain, repeat above - place yarn over hook, catch it with the hook and pull it through the loop already on hook, slipping this loop off and simultaneously forming another. To prevent twisting of chain, keep your thumb and forefinger near your current stitch.
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The chain stitch is the foundation for most crocheting.
How to make a chain - "ch"
Step 1 - Make a slip loop on crochet hook a few inches from end of yarn. Adjust yarn on left hand over forefinger. Hold crochet hook in right hand like a pencil.
Step 2 - Yarn over the hook from back to front, draw yarn through loop on hook; one chain stitch made.
Repeat Step 2 until chain is desired length, holding chain close to hook as it grows longer.
Source: Crochet Primer, Bernhard Ulmann Volume 64