When I launched my show, I already had about 100,000 monthly readers on my blog and 20,000 email subscribers to seed the early listenership numbers. That helped me sell an early sponsorship slot for 10 episodes to Freshbooks at $500/episode to help fund my new podcast. All in all, that netted me a $5,000 sponsorship up front to fund starting a podcast, which was pretty awesome at the time.
That worked for me because of where my blog was already at—but when you're starting out with less traction, I recommend getting a few episodes recorded and launching your show without trying to book a sponsorship in advance. Those that tune in and like your show will subscribe and stick around. One of the best ways to reach more listeners is to be a guest on other (more established) podcasts in your space, so try pitching yourself as a guest to go on other shows.