For a first time petty theft, depending on what county you're in, they'll hopefully grant you a diversion program, meaning that you'd be on probation for a year, and as long as you don't commit any offenses within that period then you can come back and get the case dismissed.
If they don't offer something like this, plead not guilty and ask for the public defender to be assigned to see what they can work out for you. You have nothing to lose, since they represent you for free and could only get you a better deal than the D.A.'s first offer.
Make sure to address the judge as 'your honor' and dress up as best you can too.
Let me know if you're in Orange or L.A. county, since I'm specifically familiar with their practices, having defended many clients charged with petty theft in both counties.
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