The very first thing you need to learn about crocheting is hook and yarn sizes.  A very easy way to put it is, the smaller the yarn, the smaller the hook. If you use a small yarn size with a big hook, you will have holes showing through your work.  This is sometimes something you want, like if you are making a blanket or doily.  But if you are making a doll or toy (amigurumi), you will want the holes to be non-existent so the stuffing doesn't show through.  Here are some examples of different yarn and different hooks to create different size toys.
This picture shows three clouds, all made using the same pattern.  The difference is the yarn and the hook sizes.  The grey cloud was made using two strands of worsted weight held together with a 3.75mm hook.  For the pink one I used two strands of light yarn and a 3mm hook. And then for the white I uses one strand of light yarn with a 2.25mm hook.  
This picture shows two cows, also using the same pattern, but with different yarn and hook sizes.  The bigger dolls were made using chunky yarn and a 6mm hook and the smaller ones were made with worsted weight yarn and a 2.5mm hook.
And here is a picture where you can see the differences in stitch sizes.  The smallest yarn has tiny stitches, the chunky yarn has huge stitches. So depending on what size yarn and hook you use, you can make all sorts of different things. 

Now a little about hooks. Hook sizes can be different for different countries.  For example, in the US, we use an alphabetical system.  The sizes closer to the beginning of the alphabet are smaller, and those closer to the end are larger. There is a metric style, measured in mm's and the UK and Canada use different numbers.  And in the US we use letters. Kind of annoying.  I like the metric style because most of my hooks from Amazon are metric. Makes it easy for me.  Here is an easy conversion chart for you.  
Yarn is a little more complicated and not an exact science like the hooks.  The fibers can be thicker in some areas, thinner in others, or have knots in them.... Yarn sizes are called weights and the weight doesn't mean the weight of the skein, but actually the thickness of the yarn strand.
Here is a simple chart to help you with yarn weights and sizes.


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