How do I create an artist tag for myself so I can have that little yellow link to all of my work?

If the tag is unique, the first time you tag it you can prepend artist: to it. So tag a post with artist:autumntree or artist:autumn_tree (whatever you prefer) and it will be made as an artist tag. From them on, you only need to tag autumntree or autumn_tree and it will automatically be an artist tag.

EDIT:
Though it seems the tag was already used for your posts, and someone already changed it to an Artist tag type. For future reference, if the artist was already tagged to posts but it was a General tag instead of an Artist tag, click the ? next to it on the post to go to its auto-generated wiki page, and click Edit Tag to change its type (but don't do this for a tag that's already being used for something else that's not the artist).

I really don't like the artist: method of changing tag category, since it's so situationally dependent. The "?" method works too, but it can be confusing to new users.
The easiest way to do change the category is to add the tag to a post, then go to the tag search, find the tag, and click "Edit" next to it.