You can start by writing an article on PowerShell.org ;). If you submit one to Webmaster@, we'll be happy to publish it.
Also review what's on http://powershell.org/wp/user-groups/tips-for-starting-a-powershell-user-group/. Twitter can be an effective way of reaching an initial audience, and most of us will be happy to re-tweet if you let us know about it. You can also send an announcement to newsletter@ here, and they can get it into our newsletter (there's a 30-day lead time).
Once you're up and running, have a look at http://cdn.powershell.org/wp/wp-content/uploads/2013/05/User-Group-Services.docx also.