To do so, you click on the "Control Panel" at the upper-right-hand corner of the page. It's next to your Account name and picture. Inside, you can add text to any of the fields. Adding text to the "Biography" box will have it show up on your profile page.
i was gonna make a thread like this but.... here's a suggestion.
I haven't been to AG3 beta so i don't know but i think there should be an easier way to add links and edit your profile biography. i mean, the box is way too small for wanting to write out so much stuff.