Hook Sizes and Yarn Weights

Some Crochet hooks are bigger than others, therefore they make bigger stitches. Smaller Crochet hooks make smaller stitches. Big hooks are recommended for beginners. Small hooks can be used with thin yarn but they're not really for thick yarn. Big hooks on the other hand can be used for thick or thin yarn. You should start off with a big hook and some not-to-thick yarn. Once you are comfortable with Crocheting you can start using smaller hooks and thinner yarn. I'm not saying you absolutly have to go by the instructions when you use a pattern, I'm just saying you should experiment with different kinds of yarn and different hook sizes. If you use a bigger hook than the pattern said you should the thing you make will probably be bigger. If you want to know more about yarn weights go here.
If you want to know more about hook sizes go here.