Brainstorm and make a list of all of the places where your target users "hang out".

This can include both online and offline places where you can participate in their communities. For each potential channel, research the cost (actual dollars or time) that will be required to convert users through that channel (example - the cost per click if its advertising).

Popular channels include social networking sites, content sites, real-world events (conferences, meetups), etc. Also include as channels things like strategic partnerships, marketing partnerships, and affiliate programs.