Help plan a trip to San Francisco?

My 2 friends and I were all girls in college on a budget and we are planning a trip to san fran. We live in tustin. We wanna know what would be cheaper flying or driving. We will be leaving in June. What are some rlly cheap motels or hotels. What are some things to do and what shall we see. We are going for cheap but we wanna have fun. I will be 19 but the other 2 will be 18, R there any good 18 and over clubs and how do we stay safe as we r still young thanks!