The most important thing is that your hook size and your yarn size match. If you’re an absolute beginner and unsure of where to start, go with a worsted (Weight 4) yarn and either a size 4.0mm hook if you’re trying to make amigurumi or a size 5.0mm hook if you’re trying to make a flat piece like a blanket or scarf. Hook sizes are listed in their mm measurement and also according to an arbitrary naming system that I recommend you ignore. It’s not standardized across brands, while the mm size measurement is.
If you are making a flat piece, you use the size of the hook listed on the label:
If you are making amigurumi, you go 1-2 hook sizes DOWN from the hook listed on the yarn label.
Heavier yarn creates bigger pieces and smaller yarn creates smaller pieces. Here is a line-up of the same pattern from yarn weight 0 to 6 (my small hands have a difficult time with yarn weight 7):
Here is a handy chart if you don’t have any yarn labels on you to approximate what yarn weight and hook match for amigurumi:
