Please start by entering your User Control Panel (link under the tabs), then going through to set your preferences. Help on how to use the Forum is available from the FAQs linked below the tabs, from the detailed phpBB User Guide on the Manuals tab, from the Forum How-To's here
and from the Quick Start Guide
Please be aware that as a new user, certain forum features will not be available to you until you have posted 5 times. Among them are the ability to have a signature attached to posts, and to change your email. There are also others. After your fifth post these will become available.
It is not a bad idea for all users to review these documents as a refresher.