What to start?

Where do you want your first flower bed to be? Once you choose a location, you can start digging the grass and weeds out of it.

Have a soil test done to see what, if any, vital nutrients the soil might not have enough of to sustain plant life then you can begin amending the soil as needed. Soil test kits are available at most large garden centers, your local country extension office or the Ag dept of a local college or at least they can tell you where to send off for one.
The soil test will also tell you the pH level of the soil, alkaline or acidic, that too will be a determining factor in what you can and can not plant in that bed. If the pH level is too alkaline or too acidic, then you will need to dig in the proper amendment for that too.

While you are preparing the bed keep track of how much sun and shade it gets. At least 6 hours of direct sun a day is required for Full Sun plants, then there are sun/part shade plants, plants that will like bright shade and some that require full shade. That info is on the tag/label of plants when you buy them so you can get the proper plant for the location.

Find some plant nurseries, garden centers in your area and start roaming up and down the aisles to see what plants are available and what their requirements are. Depending on where in California you are, you might be able to do some planting this fall when the bed is ready. But at least you can see what is offered in your area this time of year and ask them what they carry in the spring too.

California has such tight restrictions on what plants can be shipped into the state, you will probably have to depend on local shops for your plants which also means more native plants. And going native means less additional fertilizers, less watering and a much lower maintainence garden.

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To be interested in the changing seasons is a happier state of mind than to be hopelessly in love with Spring ----
George Santayana