1. Go to your profile page: http://www.wikitree.com/wiki/Scott-14588
2. Click on the tab that says "Edit".
3. Scroll down the Edit page. Below the data fields, you'll see a heading that says "Edit Text". Below that, on the left, is a big mostly empty box. That's where you can insert your biography. Type anything you want.
There's also some text there that says "This is the body of the profile. It's free space for for [your] biography, research notes, sources, and categories." Click on the links to get tips on formatting, etc.