It works the same as knitting- bigger hook equals looser stitches and bigger final product. You can test the difference between two hooks by making a swatch, just like with knitting- it can vary from person to person because of your tension and a host of other variables.
Also, most of the time you can use the same size hook (in mm) as the knitting needle recommendation. Go smaller for tighter stitches, larger for looser ones. Most worsted weight recommend 5.5 but I usually use a smaller one- I mainly make little stuffed things though.
If you aren't already, you should join ravelry.com
- they have lots of great tips and free projects, but it takes about 5 days to get your membership information. Also lionbrand.com
also has a ton of free patterns. You just need to sign up but it is instant.