Southern California tourist help?

3 friends and I (17-18 yr old boys) spontaneously decided we're taking a trip to Southern California... basically the San Diego area. I would like to know if there are any places we can camp on the beach, have some fires, etc. for free or for cheap? also any alternatives (hotel, motel, other camp sites) that are budget friendly would be nice. We will be doing a lot of boogie boarding so we would like an alternative close to the beach. boogie boarding won't consume all our time so what other fun, cheap things can we do in the San Diego area? I say San Diego because I need a beach far enough south that it is warm/hot any other similar beach with good temperature in Cali will work