The first thing you should do after being bitten by a dog is contact the nearest local police department and animal control unit. They will be properly trained in dealing with dangerous dogs and will investigate the incident, which may include taking statements from the parties involved and any witnesses to the attack.
The proper authorities will also likely be able to help you find out the identity and location of the dog's owner, which is a very important step for victims who want to pursue a personal injury claim against the owner(s). Then, they will also be able to help you file a dog bite report with the proper police department, which will document the attack and create a paper trail so that authorities can adequately enforce the law.
In all instances, it is absolutely necessary for all dog bite victims to seek medical attention for any and all injuries they may have suffered as a result of the dog bite. This is important because it not only ensures that your injuries are properly diagnosed and treated, but also because of the importance of medical evidence when it comes to pursuing a personal injury claim. It may even be beneficial for you (or the healthcare provider you choose to visit) to take photographs of your injuries that can later be used as evidence to support a personal injury claim.