In addition to the beaches, California has the coastal mountain range, the Sierra, volcanoes to the north, and a large desert system in the south. These add together to provide a great variety of hiking. The scene is a bit better in Los Angeles than here, but still we have good local destinations in Marin and the Santa Cruz hills, and a bit further away good times at Yosemite, Tahoe, and Shasta.

Required people - 2-. Casual urban walks can be done alone, but the further away from civilization you go, the more important it is to have a buddy. For weekend trips into the backcountry, 4 gives you more people to carry gear, and more options for separation.

Required gear - limited for day hikes, ranging to substantial for some weekend trips. DAY: good light boots, water, layered clothing. Cotton (including jeans) is discouraged due to its lousy insulation when wet. WEEKEND: more substantial boots, a pack, sleeping bag, tents, etc. How much depends on how unlike a paved trail you're hiking along.

Required time commitment - as little or much as you want. Good hikers typically average 3 miles per hour, or 1000ft of gain per hour. But don't forget to stop to enjoy the view.

Costs - once the gear is acquired, the costs of trips are quite small - limited to travel costs and some park access fees. But like many other sports, gear acquisition can be an eternal process of new toys. The boots, pack, tent, and sleeping bag are all items that range around the 100-200$ price zone, and the North Face and Marmot have many pretty fabrics to sell for a king's ransom. Nontheless, much of this is luxury spending and not necessary for the majority of destinations.

Links:

My Hiking Page - many travelogs around California.
Mt. Shasta - my favorite destination, only a 4 hour drive.

Freedom of the Hills - perhaps the best single book covering the sport.