If they are dismissive or unhelpful, complain. It's their job to facilitate public access to records. You pay them. They work for you. Researching the department can also be useful in learning what kinds of information is on file.
Every department has some sort of departmental regulations.
You should read them before filing requests. You can submit your request in letter form or simply using a basic form including your contact information, a description of what you want, and the method of access you prefer. Carefully craft your request with the intent of plucking out what you want, while discarding the mountains of paper you don't want. It's a good idea to speak with the information commissioner at the appropriate agency to get a sense of what is available and how your request should be worded. Ask for records instead of notes or reports to ensure your request covers information stores on computer disks.