There are usually 4-5 metal braces across the roof. If you have a dome light in your roof "usually" there is a brace there.
If you have a van with rear air , then you will have some trouble removing the headliner. You will need to remove the sunvisors, center overhead consoles, side trim pieces ( which usually involve removing the seatbelts), hand holders, and hangers.
It is quite complicated on some vans/suvs, and easier in some cars.