Well, it depends. Do you want to use forum software that has already been made, like SMF or IPB or PHPBB? Or do you want to write your own forum script?
If you want to write your own forum script, then it's not so hard since you can control the table structures and all.
If you are going to use forum software, then I suggest that you first install your forum, then build your game on top of the forum's table structures.
So, if the forum's `member` table contains the forum user's information, you can just add extra columns for your game information. This doesn't mean that your game will be a part of your forum, but just that they will share the same tables.