At the end of each row of a crochet pattern, it will say "turn" so you are ready to begin working on the next row. To turn the work, you do exactly that. The correct way to turn is to turn the work around so that the side that was away from you now faces you and the crochet hook is now at the right-hand side of the work, not the left.
Before you start the next row, you need to raise the crochet hook and working loop up to the same level of the stitch at the beginning of a new row. This is done by working what is called a turning chain. The number of turning chains needed will vary depending on the type of stitch being worked.
It is important to remember that you will never work into the first chain from the crochet hook unless your instructions state otherwise. Depending on the stitch, you will work into the second, third, fourth etc., chain from the hook. The instructions will always state which chain you will work.
Slip stitch: You will work 1 turning chain.
Single crochet: You will work 1 turning chain.
Half double crochet: You will work 2 turning chains.