The US doesn't have anything like the Scandinavian right to roam (Swedish allemansrätten, etc.). Private land is usually fenced, and it's against the law to enter private land while hiking without the landowner's permission.
The US term for wild camping is "backcountry camping" or "backpacking," as opposed to car camping, where you pay to park your car in a demarcated area of an organized campground, next to other people.
The US has a complicated system of government lands that are protected from development and that are accessible to everyone. These include national parks, national forests, and state parks. Nicasio is in an area called Marin County. If you look at a map of the area, you'll see a variety of wilderness areas, such as Point Reyes National Seashore. If you want to camp, you need to go online and look up the information for the specific area. For instance, information for Point Reyes National Seashore is here.
Is it generally desired to make fire? It might fend off predators, but could offend land owners?
Basically, don't build fires. The wilderness areas of California are delicate and have large numbers of human visitors. The days are gone when it was acceptable to have this kind of impact on them. Conscientious backpackers these days follow the philosophy of "leave no trace." There are specific regulations in specific areas, but basically the landscape just can't handle being picked over by humans for firewood. 2014 is also one of the driest years in the last century, so the last thing we need is people starting forest fires.
I've started a separate question about dangerous wildlife in California.
The area you're going to is extremely beautiful. Have fun!
