Build a Website similar to change.org

I need to build a website similar to [login to view URL] where people will be able to file petitions and other people can sign them. It will have categories for e.g.:- technology, nature etc and will have subdomain accordingly. We will also need a database of authorities all over the world for various things to whom we can direct these petitions. Write "Petition Application" in your bid and in your description mention how do u intend to complete this and you approach. I don't need to know your current work I need to know how will u do this.