Attorneys are allowed to sign subpoenas, but if you're representing yourself, you will have to get the subpoena approved bu a judge and stamped by the court before you can serve it.
Certain law enforcement records, including an officer's disciplinary reports, are protected from disclosure. In order to get access to those records, you have to go through a complicated hearing process, where the law enforcement agency's attorney submits the records to the judge, who will review them and decide what, if anything, you get.
Please understand that this is a general discussion of legal principles by a California lawyer and does not create an attorney/client relationship. It's impossible to give detailed, accurate advice based on a few sentences on a website (and you shouldn't provide too much specific information about your legal matter on a public forum like this site, anyway). You should always seek advice from an attorney licensed in your jurisdiction who can give you an informed opinion after reviewing all of the relevant information.