If you’re going to do any work on your car’s electrical system, you need disconnect the battery before you do anything. Remove the Negative cable first and tie it out of the way, then remove the positive cable. The negative side has a minus sign and the positive a plus sign. Why do it this way? Removing the negative is removing the return path for the current so nothing can touch the terminal and cause a short. When reconnecting, put on the positive first and then the negative. Make sure the terminals are clean and tight. While you are at it, it is a good idea to coat the terminals with grease or spray paint to protect them.