Matt, I live in MDR so I am fairly familiar with the local stores. You've got REI on Rosecrans in MB, Adventure16 in WLA or Torrance (PCH at Crenshaw) and Sport Chalet at Village Del Amo. Between those three, you'll find a pretty good selection of packs and everything else. There are some specialty items you won't find anywhere but online, but for a beginner, these three stores will have most everything you need.

Go look at them, but buy all your other gear first before buying a pack. You should buy a pack to hold your gear, not buy gear to fit the pack.

Getting a pack that fits right for you is most important-there are plenty of packs in all different sizes, but they don't all fit the same body type. Some people can get away with a small pack for a 3 or 4 day trip, depending on the weather, but for most people, I would think a pack around 3000 ci would be fine. If you are just going around SoCal, the weather won't be too cold unless you head up to the mountains, so carrying a lot of winter gear probably won't be an issue.

You can find cheap packs at Sport Chalet and sometimes they will work out fine, but buying a good pack is always a good investment.