Paying for it...
Many here scoff at the idea of government funding for big projects, but that is what is often needed to make needed projects happen- The trans continental Railroad was heavily subsidized by the government, as was the interstate highway system... both of those were needed, both of those were important, both were huge projects.
I think that exploration and colonization will HAVE to be supported by the government to get started- its fine to say it should only be done in a capitalist environment by corporations trying to make a profit. The fact is work and money are required to MAKE it profitable to go to space. We humans created the concept of government because, sometimes, only a government has the power to do what is needed...