What should I add first?
Start with the things that would be hardest to replace or prove you owned: electronics, appliances, furniture and anything with a serial number. You can add the rest a room at a time.
Home inventory
A home inventory app that knows what's in every room
Fixture is a home inventory app for UK homes. Add each room, then the things in it: a photo, the make and model, the serial number, when you bought it and what it cost. When you need the list, it's there as a PDF or a spreadsheet.