What's your work like? Do you need to carry work clothes/shoes? Can you shower? Do you need to carry your lunch with you? Will you be riding in the dark sometimes/often/always?

These are kinds of things to think about when you start commuting.

I commute on a recumbent bike (and I'm thinking about converting to a trike for the winters). I currently have a small seat bag, but my winter gear is a bit bulky to carry, and I'm going to upgrade to a slightly larger trunk. I also keep my dress shoes at work so I don't have to carry those every day. When I lived in Az (probably more like your weather, depending on what part of CA you're in), one of the first things I did was get a decent rack and panniers, because I hated getting to work with a sweaty back from my backpack.
When I was commuting in AZ, I had a shower available, so I would wear my bike shorts (and little else, other than gloves and a helmet) and just carry my work clothes with me. When I had a ~2 mile commute, I would just wear my work clothes, but change for the ride home and take a long way.

Also consider how often you need to be prepared for rain where you are. In the summers, I don't care, and just get wet, but in the winter, that cold rain can really cause problems, so I had to invest in some good rain gear.