Legal Question in Business Law in California

Yes, it's legal, and yes, as with any legal business, you'll need (at least) a business license from your city or county. Before going into business, you should probably buy and study one or more of the (usually, paperback) self-help guides to starting and running a business. You sound like a raw beginner in the world of business ownership and management. It's actually rather simple, but if you have no background or experience in the world of business ownership, you'd be a doggone fool not to buy and study a couple of basic, simple, beginner-level books on "how to start and manage your own small business." There are a number of publishers, but I often suggest starting with those published by the Nolo Press.