1and1-specific information:

Go to your 1and1 control panel and under "Contract Selection" select "Select Contract". The preceding was the funniest sentence in this paragraph.

On the left-hand control panel, click on "Web Space" and from that menu click on "MySQL administration." If you've already set up your database (you should only be able to have one), it will display all of the info you need to install phpBB (or, preferably, invisionboard). If you haven't set up a database, then you'll have to go through the back-breaking task of clicking the "Set Up Database" button.