Well, you will have some learning to do, to be sure, but "first things first".
Do you have all the account credentials for this site? By that I mean:
1) DH Account user name and Password
2) Dmain/site FTP or shell login user and password
3) PhpBB Admin user name password
Unless you have all those things, things like "changing the front page" or modifying the site in anyway will not be possible, so yo need to start there. Once you have all that sorted, you can proceed.