If you sell things, generally you need to collect sales tax.  In California you’ll need to talk to the Franchise Tax Board.  They have a customer service site, that’s being fairly useful, though not great. The link to information on getting a permit is here.

The site tells you about the documents you need. You can send your application in the mail or you can go in to the office to do the filing.

My experience is that many government offices, especially around taxes are fairly nice, helpful people who are willing to show you what you need and explain the process.