OK, let’s take a look at cour code
{exp:weblog:entries weblog="my_weblog" oderby="ratign_avg" limit="15"}
Content
{/exp:weblog:entries}
Even with the spelling error taken into account (thanks for catching that one, joelene), you are not using the rating module at all: you are, in fact, trying to use a parameter in the exp:weblog module. I am little unclear what exactly you are trying to achieve, but if you want to simply order by rating, from higest to lowest, you could try something like this:
{exp:rating:rankings form_name="formname" precision="1" thousands="," fractions="." limit="15" }
Let us know if you had something else in mind. A good look at the docs should make this much clearer to you.