I understand how difficult such a class can be especially for a new teacher. Please don’t doubt yourself or lose confidence in yourself; I’m sure that experienced teachers would also have difficulties with such a class.
I’ll focus on the question of discipline because that is crucial here. I’ll answer your other questions later and please write in again, if you want more ideas. Discipline (or classroom management) is important in all classes and even more so in large classes. Establish rules with the students at the beginning. This will include speaking English, speaking quietly, waiting for their turn to speak and anything else you can think of. Let students know what will happen if they don’t follow the rules. Have a scale of what will happen: e.g. 1) teacher speaks to student immediately; 2) student is asked to move to another part of the class; 3) student is given extra homework; 4) teacher speaks to parent; 5) student goes to see head of school. Work out a scale that will work for you and your situation; let students know what it is and keep it in front of you at all times. This will remind you what to do when something happens and will prevent you from over-reacting when a student breaks a rule. Remember to speak to the head of the school if you get close to step 5.
One thing that can help manage difficult classes is to establish routines. This could simply be that you use the same structure to your lessons. For example, start by checking homework and/or a warmer, then move onto quiet work (reading, writing, etc) and then onto speaking. You can vary this when their behaviour improves.
Always wait for students to stop talking before you start talking yourself. Don’t speak while others are speaking and definitely do not start speaking louder in an effort to drown out their voices. Sometimes speaking more quietly can have the effect of silencing students. Occasionally, go and sit or stand at the back or middle of the class and address them from there. Don’t let clusters of rowdy students gather at the back of the class or anywhere else.
It is important to have a repertoire of activities for settling students. Exercises that help them settle involve reading, listening, answering questions in their workbooks (give a challenging time limit so that they stay focussed). Always take such exercises into class even if the focus is speaking. If things get out of hand, just change the speaking activity to your settling activity.
It’s important to decide at the beginning how you will deal with students who finish quickly. Have something up your sleeve for students who finish early; this could be exercises from their workbook, doing homework (make it sound like a treat; i.e. students don’t have to do their homework at home). You could have an early finishers corner where there are exercises and / or books. Make sure that students know beforehand what they can do when they finish; this means you won’t have to explain it repeatedly.
It’s also a good idea to turn up early to class and stay around a few minutes at the end: this will give you a chance to speak to students, get to know them and will allow you to tell them what they are doing well and what they are doing badly. Please remember to praise all good behaviour.
I will put in another post describing more draconian measures which can also work but try out the ideas above first.