It isn't easy starting from the bottom: a number tech CEOs and founders spoke on stage in San Francisco this past week for Disrupt SF 2013, one of the biggest tech conferences of the year. From Yahoo CEO Marissa Mayer, to Twitter CEO Dick Costolo, many of them gave some insight into their beginnings, the challenges they faced, and how they battled through them. Some started a company from the bottom up, and some took over a company while it was in trouble, but all of them understand that it's a new world out there, and it's going to take some tough decisions in order to change it. Now that Disrupt SF 2013 is officially over, here is the best advice given by CEOs and founders at the conference.