Site selection is extremely critical, since solar panels take up space, and space is a premium in California. Most California homeowners can't afford to simply clear out a field and plant an array of solar panels there. We have to work with existing structures, existing trees, and to be cost effective we usually must mount the panels flush on the roof surface.
The first step the pros usually take is to look at an aerial photo of the property on google maps. The best mounting locations are on south and west facing roofs. Shade is enemy number one, so look for tall buildings and trees that might cast shade on the spot you picked out. The rule of thumb is that the object should be at least twice as far away as it is taller than your spot. So if your roof is 10 feet tall, and your neighbor's tree to the south is 20 feet tall, it better be at least 20 feet away! Otherwise that site is automatically ruled out, pick a different roof surface. The critical time to be 100% shade free is between high noon and 6pm in the summer months, as this is when electricity is in highest demand and is thus the most costly. So look for shade objects to the south and west of your site in the aerial photo.
I can't stress enough how important it is that the spot you pick be shade free. Even a small shadow from a power line or a tall trunk of a palm tree can be enough to basically wipe out production of a full string of panels. It has to do with the way the panels are electrically built: bypass diodes inside the module will cause the production to go to zero in a shadow.
Installation is also possible on flat roofs and patio covers, but I would consult a structural engineer to feel confident on anything other than an existing pitched roof with shingles or tiles over 2x4 or similar rafters.
Now that you're through doing the initial work on google maps, and are fairly confident you have a spot (or a few spots) that are decent candidates, it's time to get up on the roof and take a closer look. On a sunny weekend, go to your candidate sites at 9AM, noon, 3PM, 6PM and note where you see shadows falling. Avoid those spots that catch shade as much as possible! If there's too many shadows, eliminate that candidate and move on to the next one.
In the next step I'll go over advanced site selection, and we'll break out the tools that will help to really solidify what makes a great site.