To add a small bio about yourself you need to head to 'Settings'.
On the website you do this by clicking on your username in the header bar. Click on 'Your Profile' and there is a section called 'More about you:' Add what ever you want and then save the changes using the button provided.
If your using the App click on the 'My Profile' Link at the bottom of the page, then click on the cog icon to open up the settings menu and choose 'My Profile'. Click in the 'Biography' field to open the edit screen and again, save changes when happy.