Start with your web hosting provider. Some hosts have Forum plug-ins you can activate in your server's Admin or C-Panel. If no such feature comes with your hosting, you'll need to find out which server-side programming languages and databases your server supports. Also make notes about the versions of each as this will effect which Forum solutions you can use.
Then figure out what features are most important to you:
- Price range,
- How many forum members can it support at once?
- Automated or manual Sign-up process?
- Can it be customized?
- Emoticons,
- Avatars,
- WYSIWYG editing,
- How complex is the set-up & maintenance?
- How about tech support?
- Etc...
Nancy O.