How do you make a fanfiction site?

Photoshop

Well, majority of people use Photoshop now-a-days and getting a free download is easy to find if you look in the right places.

Once you've got PS (Photoshop) downloaded and situated, I suggest that you google the basics of the program.

Next thing I would do is YouTube or search on Tumblr some PS tutorials. Then you'll start to get the hang of everything and will be able to make your own banner for your fanfiction.