If you have an attorney, your attorney will request the video during the discovery process.
If you do not have an attorney, you need to get one.
The first stage in a criminal prosecution is a preliminary hearing. The Rules of Criminal Procedure do not require that the police turn over any evidence to you or your attorney prior to the preliminary hearing.
At a preliminary hearing, the sole issue is whether the police can establish a prima facie case. In laymans terms, this means, "Is it more likely than not that a crime was committed and is it more likely than not that the person charged is the person that committed the crime."
For a typical DUI case, a prima facie case is easy to establish. Were you (1) operating a motor vehicle, (2) on a public highway or trafficway, and (3) when tested, was your blood alcohol content above a .08%. If you refused to submit to a breath or blood test, your performance on the field sobriety tests and driving behavior become more relevant.
A video is useful for the purpose of suppression of evidence. In other words, did the officer lack probable cause to stop your vehicle or did the officer lack probable cause to arrest you. A motion to suppress evidence is filed in the Court of Common Pleas. The motion is filed after a Request for Discovery.
After your attorney requests discovery, (and in Lancaster County, a specific request for the video along with a blank DVD), he will review the video and determine whether he will recommend filing a motion to suppress evidence.
Does this sound complicated? That is because it is. This is why you need an attorney.
Mr. Morgan has outlined the steps well ,I would add however, that sometimes video tapes are destroyed or "unexplainably" lost especially if they support the defendants position. It would be a good idea for your attorney to immediately send a letter instructing the police to preserve the tape which can be obtained through the criminal discovery process at a later date.