Let's start at the beginning with describing the process. I adapted this from the book, [amazon-item:0874777461|Creating Community Anywhere].
When forming a community, you are bringing a group of people together for some activity. What is that activity, and why do you want to do this? Do you have some goal you want to achieve, such as political activism? Do you want to be the host/hostess of an ongoing social gathering? Do you want to foster discussion around a certain topic area?
Just as with the process for starting a web site, with forming a community one starts with the purpose you see for the community. Let the form, location, infrastructure, and everything come from that purpose.
Note how the process doesn't mention a thing about the location. Let me remind you, the building (or web site) where the people meet is not the community. The building (or web site) where ones community meet is not the community, nor is a town or a city necessarily a community, regardless of how people speak of these things. It is the relationships between the people which form the community.
Since the topic here is online community, you are going to have some kind of web site through which the community members meet. They will conduct the community activities through that web site.
A choice you must make early is the sort of software to be used in running this web site. Different website software is suitable for different sorts of community activites, hence the purpose of your community will determine the type
of software. Therefore an early step in forming the community is to choose the software for the website. Of course, a closely following step is to install and configure that software.
Once you have the web site software chosen, set up, and configured, comes the process of finding your community members.
This can be just like promoting any other kind of web site. You will have chosen a topic area the community web site is to cover, so if you start writing articles matching the topic area, the search engines will automatically start sending visitors your way. This might be slow, so you might start advertising. Or you might purposely reach out such as contacting other web masters to exchange links, making postings on other forums, etc.