Whenever you start you first class, you need to get them hooked right at the beginning. If you want to set any ground rules, then lead as you mean to go on. I have found setting a few basic rules integral for establishing any sort of organised classroom. For example, I have a whistle to hand if required but a few rhythmic claps to the hand and getting students to repeat the same clap organises learners straight away. If you want to get learners focused and ready for class, train them well and they will be ready for what lies ahead.