Figure out what your specific niche will be.
Since this will be your forum, it is best to pick something in which you are interested and knowledgeable. Since there are already so many forums out there, it is best to have a defined niche. Something well-defined will help your forum get noticed by the search engines and also help attract your audience. You do not want something too general or too common in forums. You can always add a "general chit chat" area for any off topic business.
Choose a forum script.
There are many different forum scripts available. It is best to start with a free forum script. Make sure that it is able to move information to another forum script should you want to change later. A good free script that does allow you to transfer your information is phpBB. This is the script that many web hosts offer as part of their packages as well. When you want to change to a paid forum script, vBulletin is a good one and it comes with a migration tool for phpBB, which will allow you to easily transfer all your forum information without losing anything.
Find a good forum hosting program.
You can choose to set up your own message board using software if you have experience with web development. If not, you will have to have a host. You will need a good host that does not have many problems or downtime.
Start creating your forum or discussion areas.
You will want to start off with about 5 to 10 forums. They should all be related to the main topic of your board. You do not want to over categorize your forum as it many confuse and put off users.
Advertise your forum to get users.
Once your forum is up and running, you will need to have users to make your community. If you have friends that are online and have similar interests, you can e-mail or instant message them to let them know you have started a new forum. If you have your own website or blog, you can make a post to announce the grand opening of your forum to your readers. If you are already active in other forums, you can add a link to your forum in your signature. Search online for other forums that are related to your niche. Start contributing to discussions there and add a link to your signature.
Make posts to your forum.
As the originator, you should be making posts all the time. Respond to posts by your members and start topics that will provoke conversations among the members. If you have members that are active in your community, you can have them be moderators so that they will be encouraged to stick around and post more.