Like furnace says, basically when you go to the site for the first time it usually asks you for details such as the database server, name, username etc. The table prefix is usually one of the questions. It can be anything of your choosing however, I'd pick something like "phpbb_" or "forum_" or something to that effect. Make sure that you use an underscore or something in the middle though because the tables will come out like "forum_users" etc, without the underscore it looks more of a mess, for instance, "forumusers".