You need to do the following steps,

1) Go to the sheriff office, pay some money to check your crimial history, they would issue you a document showed an arrest record but no charge.

2) Go to the gneral attorney's office to get the document of your case. But if no case, there would be nothing in their office.

3)Go to the Court, provide your ID and they would give you a paper showed arrest was made however no case was docked.

Any one of these papers was offical document USCIS would accept.

Good Luck!