Let's start with this. The blog linked to your username produces a 404 - page not found. Please post the link to the blog in question starting with http://
The default category for all wordpress blogs is "Uncategorized". A default category will be assigned to any published post in the case you don’t specify one. If you wish to assign another one to your posts and designate it as the default category then you can do so. This default category can be set under Settings > Writing.
However, until we have the correct link to the blog in question we cannot view the blog and provide any specific help so please post it.