SF is crazy expensive. When I moved hear I found a cheap room on Craigslist for $800/month. Now they are more but still possible to find. Walnut Creek is pretty far to commute from and I think not that cheap. I would focus on finding a rare but affordable place in SF or looking at Oakland / Berkeley. Hope that helps!
I was a Poly Sci major from UCLA and started in Sales and now Marketing for the past 8 years. Your undergrad degree matters less than your work experience, hustle, and aptitude. At undergrad you should be learning how to solve problems and break them down into manageable chunks which is similar to what you'll face in the working world every day alongside how to work with a diverse group of people. For marketing, start refining your own taste of what you think is effective or not and add on data to make it more objective.