The key thing to good classroom management is to have a well-planned lesson. As a cover supervisor, you may get rubbish from the regular teacher.
If at all possible, get hold of the work ahead of time so you can either discuss it with the teacher or modify it yourself. If the work is to 'read from the text book and answer questions', make this as active as you can - get them to take turns reading alound, and answering the questions aloud, before settling down to write down the answers in sentences. If you have 'silent reading' in English, get the pupils to come up to you one by one to read alound - they love doing this up to Year 8 or so.
If you are basically supervising silent work, do it yourself so that you improve your knowledge which will help you in the future - it is not that hard to be one step ahead of the pupils.
You will be on a very steep learning curve - even after one week you will know tonnes more than when you started. An important thing is to establish a good reputation, which basically means blagging your way through the first week or two.
Easy things you can do is to have a class list, and seat them in register order (unless the regular teacher has a seating plan that you can use).
If you have to 'discipline' anyone, follow a procedure, eg explain how you expect them to behave and warn them of the consequences. If they continue to misbehave, then follow up. The school should have a clear sanctions policy that you should follow, eg to involve Head of Department or duty Senior Manager.