First of all, turn off your water. It’s best to turn it off at the house level. This means that you won’t be able to use water anywhere in your home, but it also means that you won’t have excess water flowing into your home. Your shutoff valve is probably in the basement near the rest of your utilities, outside by a spigot, or in your garage. Ideally, you will know where it is before you need it. If you don’t know, though, look around. It’s usually red and has some sort of marking. Assume that it is on and turn it however you need to turn it to get it off. Pretty soon, the water should stop flowing from your broken pipe.
After that, give us a call. Unless you know plumbing, the best way to deal with a broken pipe is to call in a professional. Your plumber will be able to locate the leak, let you know if there are any more, and get everything fixed for you fast!