San Diego as a city is actually great for night life! However, you have to be 21 to access many parts of it. Still, there are a decent number of clubs and the sort for 18+.
I know tons of people who party, and constantly. I live in the Muir college apartments and I literally hear people partying every day. I also know people who never leave their rooms. That's the thing about college, it all depends on what you want to do. If you're a social person I can assure you you will find people who share the same interests as you.
But that being said SD isn't the type of school where parties are going to come knocking on your door. You will hear this from everyone, but BE PROACTIVE. Fortunately, that's going to be really easy during the first few weeks of school when nobody knows each other. And if all else fails, join a frat/sorority. I can guarantee you, all the Greek people I know are very social. Some are even as social as the ones in UCSB.
There are concerts on campus several times a week. You can check out the Loft or Porter's Pub. There are also tons of concerts in the San Diego area (but keep in mind UCSD is in La Jolla, not San Diego).
Whatever you choose I wish you the best of luck!