As soon as you create your account, you will receive an e-mail with your password to access the system. You can change it anytime by clicking on your name at the top right of the screen, then click "My profile".
Choose a new password and click "Save" at the bottom of the page.
In this e-mail you will also find a link to validate your account. Just click it and you will be redirected to your Runrun.it home page.
Now your account is validated!