Plant selection comes last.

Unless you're a plant nut, in which case you choose the plants, and then decide how to accommodate them. But that isn't landscaping (it's plant collecting), and doesn't always yield the best decorative or functional outcome.

So, your first list is of what needs the space will serve. This can range from privacy, shade, and playspace to mail carrier access, compost, movement around the property, or drainage. It is perfectly legitimate to have a need for colour, early action in spring, fragrance, or other plant-related needs, but still, identify the need first.

Even if it's just about looking good, there will be some sightlines to identify and thoughts about when you want it to look good to whom. Sketch out tree locations, bed shapes, plant heights, and think about what characteristics those installations can/can't have: consider tree debris, tree roots, deciduous vs. evergreen, maintenance requirements, respect of boundaries.

Then you can go to the nursery :-)

KarinL

Not much info to go on, but I also live in NorCal so I'd suggest first, you sketch out a plan. You obviously have some idea of what you want out of the space - a place to sit? Vegetables? Flowers to cut? A year-round vista for a window?

A sketch helps for a variety of reasons. First of all, you want to consider not just a front-forward view, but all angles that this bed might be seen from. You also want to consider that we have a full four seasons of interest in which you'll want to have your bed look attractive most, if not all, of the year.

You want to consider height, width, foliage color/texture/size/shape, and LASTLY, bloom time and color, for every plant you'll need.

Pick your 'structural bones' plants first. This can be whatever plant you are lusting after, that you want to be the highlight of your bed. A stunning specimen Japanese maple, perhaps, or a lusciously scented lilac. Whatever it is, the remaining plants need to contrast, yet also complement it in size, color and shape.

The biggest mistake newbies make is forgetting that leaf shapes should contrast for visual interest. There is nothing more boring than a stretch of velvet lawn broken up by a row of plants that are exactly the same color green as the grass, without sufficient texture or shape to offer any relief to the eye.

Figure out your watering needs. With the best will in the world you cannot make lavendars like summer water; it will shorten their lives even if you give them good drainage. Ceanothus and certain others will just give up and croak if you even try it. If you get fog, putting in David Austin roses is guaranteed to break your heart. Will you use soaker hoses or drip irrigation? Automatic timer or manual? How will you weed (e.g., where will you step)? Are you willing to mulch?

Now you've got the beginnings of a plant list. Great, but guess what - there's only 3 wholesalers supplying every single big box garden center and high-end nursery. So they pretty much all have the same plants at the same time, because everybody's getting them from the same sources. So your chances of getting all the exact plants you want at any given time, are actually pretty slim unless you've got a short list of very common plants.

So....you need to be willing to give this a little time, buying the plants you want as they become available. You can always special order, but it costs a lot (I've done it, and seeing a 5' tall $250 tree croak was not a happy experience). And you might find something unexpected that will be Just The Right Thing to substitute for something more common, as you happily peruse the nursery aisles on a semi-regular basis.

I'm not a pro, but anyway, that's my suggestions and HTH!