In order to handle noise in the classroom, it's important to set expectations so that students know when it's OK to talk and when they should be quiet. Make it clear to students when they're free to socialize and be loud and when they are not. For example, you can tell them that before the bell rings or during group work they are free to talk, but being quiet is necessary when the teacher is talking or when they are doing individual work.
If the noise level in the classroom gets out of control, the next best way to handle it is to use cues to get students' attention and alert them to the need to be quiet. Start whispering or talking in a low voice, saying that you would like the students to stop talking and pay attention. Eventually, the students will realize they need to stop what they're doing and listen. Use a bell or gong or start playing music in order to get students' attention. Once everyone is quiet, remind them that your cue -- whatever it is -- means they need to start quieting down.
If the whisper technique does not work, give out warnings to the entire class or individual students. With younger students, you may want to allow two or three warnings before taking disciplinary action. With older students, one warning should send a clear message. You can implement warnings by writing the student's name on the board or simply giving a verbal warning. If you're going to warn the whole class, try to make sure you give out warnings only if the majority of the kids are being loud, so as not to punish students who are being quiet and respectful.
If students continue being loud in the classroom, it's necessary to use disciplinary tactics to handle things. If there is a group of students being disruptive and loud, discipline those students only. If you feel the majority of the class is not following directions or being noisy, extend your disciplinary tactics to the entire class. You can shorten students' recess time, make them do extra homework or send them to the principal's office. Try different tactics and see which ones work best for your class.
In some cases, you might need the help of another teacher or your principal to deal with a noisy classroom. Ask your principal to come talk to the children or help you brainstorm better ways to handle the noise. Children may respond better when a figure of higher authority tells them they need to behave. You may also need to ask for help if certain noisy children have learning disabilities or behavioral problems that affect their behavior in the classroom.