What you said
Year 9 are renowned for being a challenging group. Try speaking to their form tutor for strategies; talking to individuals; speaking to their previous teacher; brushing up on the behaviour policy, contacting parents and getting some behaviour agreements in place; and speaking to the head of department about where to go with the class. You should not be tackling this alone.
The expert view
Never fear, this sounds like what every teacher goes through. You have a new group and children are often frighteningly resistant to change. And they are Year 9, a group that appears hard-wired to refuse your simplest instruction. The problem isn't you, it's them, and how they respond to new adults.
Set your stall out: tell them exactly what you want in terms of behaviour. Don't bother negotiating the rules. Just tell them what you need from them in order to run an ordered room. Definitely have a seating plan and if they object, then to hell with them. Summon other staff to assist you in getting them in their seats. This is not playtime and they have to sit where they learn best, not according to their friendship groupings.
Anyone who mucks around or ignores instructions, give them a detention. If they reoffend, muck around in detentions or fail to show, then repeat the process, but this time get line managers involved, escalate the sanctions and call parents. Then repeat, repeat, repeat. It will exhaust you. But it's an investment in your own future.
Tom Bennett is author of The Behaviour Guru and Not Quite a Teacher. Read more from Tom on his TES blog, or follow him on Twitter at @tesBehaviour. His latest book, Teacher, is out now, published by Continuum. Post your questions at www.tes.co.ukbehaviour.