Every state has laws establishing which official records may be disclosed and which may be (or must be) kept from disclosure. These laws are referred to by names such as "Public Records Act" or "Freedom of Information Act." See e.g. California Public Records Act at Government Code section 6250 et seq.
There may be other laws which require disclosure of records, such as during civil or criminal litigation, or as a function of the rights of a victim of crime. Additionally, it may be the custom of a local agency to disclose records upon request, such as with animal control agencies which provide a copy of their file to the victim of a dog attack.
To obtain a copy of records under your state's public records act, call the agency that you believe has the records, and simply ask for them, telling them your connection to the matter. On that call, they will advise as to whether you have to fill out a form and pay a fee. Follow their process and you should have no problem. Do the necessary well before any court appearance or other event, because agencies might take weeks to comply with your request. If you are a crime victim or have a low income, inquire as to a fee waiver.
Even if the agency has a legal ground to refuse to disclose records, most records can be disclosed in the discretion of the agency. See e.g. Top 10 Points to Remember about Exemptions to the California Public Records Act by the Center for Public Forum Rights.