Up in the upper left corner, you can see a button called "Contribute". Click it, and you get a drop-down menu of options like "Create a Map", "Add a page", etc. Click "Add a page", which will bring you to Sonic Fanon Wiki:New Page.
Here, you can make various pages for things like characters, items, locations, etc. Since you're making a character, I would suggest entering that character's name into the little box in the "Character Articles" section, and clicking create.
Once there, you'll be able to write in things like your character's appearance, personality, abilities, and a lot more. They'll need to meet standards, though - you can read them here: Sonic Fanon Wiki:Policy/Standards#Characters
Let me know if you need anything else, okie?
...upper right, upper right, yeah. Man, I always get my directions confused TnT
As a second option there's also the "Add Stuff!" tab, which was what I always used. Yay for options.
Yeah, that too. .o.