It sounds like you haven't been given much information?
Can you email the office to see if you can get an overall department timetable.
Also, I would prioritise finding out what the new, incoming Y11s have done - it may not be IGCSE but they must have done something? Then you can work out what they need in Y11 if there are gaps that need filling.
So those two things need to be a priority.
I would then suggest working out what you department priorities are for this year and produce an action plan.
So, one could be preparing for the new IB
Teaching and learning focus- challenge/oracy/literacy??
Etc.
For the first few weeks, I would strongly recommend really getting to know your team and the department- what are the strengths, areas for development, what works and what needs changing? This then feeds into the development plan.
Make sure you meet your line manager early in the term to discuss priorities and the action plan.
Are you a new HoD? If so, find someone in school who is experienced at the role and see if you can buddy up.
Good luck!