Tips for success
1. Content
Like I, and everyone else has said before, content is king. If you don't have content, don't expect your visitors to stay, if you even get any. If you have a blog, adding content is easy, just keep submitting new articles. Same thing goes for a forum, it may take a little time, but it is worth it at the end. Once you get a good amount of content, advertise.
2. Making people interested
The next thing to do would be to find things that attract people to your web site or forum. Make it unique, something that the other web sites do not have. Another good thing that would attract people would be to give out prizes, like domains, VIP access, special profile features, and other things. If you do not have any of this, your users will eventually get bored, and become inactive, or even worse, leave the site to go looking for another site with the same topic.
3. It's all about the looks
No one wants to look at an ugly web site, "theme", or style. Try making a unique theme for your web site, if you do not know how to code, you can always find free ones, or buy one.
4. Expanding your site
Once your web site has enough members, I really couldn't give you an exact number, but you should offer them new features to keep them interested. For example, you could offer them their own blogs, a store, and if you have a forum, a simple money or points system. This may not be much, but it should keep them interested.
Well, that's all my tips. I hope this helped some of you!