To change the headline at the top,
1. Click "Administration Control Panel" at the bottom of the page.
2. Log in with the email and password you provided when the forum was created.
3. Under "Board Configuration" on the left, click "Board settings".
4. Change "Site name" and Site description" to your desired values
The administration CP is also where you go to manage users, groups, and forums. I recommend browsing around in it and experimenting with the settings there.
This article describes how you'd go about installing a new style for your board: http://www.phpbb.com/kb/article/how-to- ... on-phpbb3/