Below is the link of California state tax calculator provided by FTB on california government website.
Choose the filing status and provide the gross income to calculate your state taxes. You can estimate your refund as per your taxes that you paid on your income.
You can file your california state taxes free of cost using government e-file options. You can read more on http://fstax.blogspot.com/2010/01/file-your-simple-california-state-tax.html