I want to remove the category from post slug, for example a post slug like "http://my_site.com/uncategorized/hello-world/" would become "http://my_site.com/hello-world/".
My search only gives results about how to remove the word "category" from slugs, which is a different problem.
As @E pointed out, this is handled in Permalink Settings.
To access these settings, look in the Admin menu on the left side of your screen.
Under 'Settings' click on 'Permalinks'
In 'Permalinks' you can see all of your various options for how to format the permalinks.
It sounds like you want to use the 'Post name' option. Choose that and 'Save Changes'
That ought to set you right.
