Notice how you have selected your demographic in this way; persons interested in the same song/movie/book as you. In addition, if your site is the only one that presents such information it will eventually become very popular because only your site will present an opportunity for these like minded people to get together.
When creating your forum, you should avoid:
1) Bad site design. Even if you are not a graphic designer be sure not to include lots of ads and flashing gifts on your site. This makes the site look like a spammer site. The simplest design can be best even if it does look a little awkward; at least it won't be annoying your visitors. You will loose a lot of visitors if your design is bad and there is an alternative site with a reasonable or better design.
2) Poor moderation. By this I am referring to a number of things that can affect the growth of the forum.
These include users posting vulgar or spammy or even abusive stuff, having inconsistent rules; one user may be penalized for something while another might not for the same reason and banning a person because of one mistake. All these are classified as poor moderation and unless your forum is very big with lots of posts everyday, poor moderation can cause users to disappear and find better moderated forums.
3) Nuking the board. As times goes by you may be required to reinstall the board but doing so often and having the users re-registering every time you do this can cause you to lose your users and thus overall forum growth.
And when creating the board, do the following:
1) If you have a fan forum, try and locate the original artist of the work you are amazed by and check if he/she will link to you. Always be polite when asking. If they agree then traffic from the original sit will soon drive to your site. You can see how well this works in book fandom.
2) Advertise discreetly. While it is OK to put links to your site in your profile on other boards it is not advisable to post ads on their boards. To do so is disrespectful. If persons are fond of you they will take a peek at your profile and may come to visit your forum.
3) Post interesting topics. You will not get visitors if your forum is full of spammy topics which are boring. If you carefully write a post to get discussion/opinions going then persons will contribute. Far less persons will start a new topic unless they are comfortable on your forum or they are highly sociable.
4) Try to keep the good posters you have. It is always important to cherish the posters that keep the forum alive and growing. They also lessen your workload as you do not need to keep posting all the time and also more persons become attracted to your forums because of this.