Go to icloud.com/find from any browser. Sign in with your Apple ID. Select your device, then tap Mark As Lost. This locks it, disables Apple Pay, and shows a message with a contact number on the screen.

From the same icloud.com/find page, select your device and tap Erase iPhone. This wipes all your data remotely. Your phone will still be locked to your Apple ID even after a wipe, so a thief can't use it.

Report it stolen so they can block your SIM and stop anyone making calls or using data on your account. Ask them to blacklist the IMEI too.

Go to appleid.apple.com and change your password immediately. This signs the thief out of iCloud, iMessage, FaceTime, and the App Store.

Email, banking apps, social media. Anything you were logged into on the phone. Do banking first.

This is more likely to happen when you're travelling. If you're in the UK, report online at police.uk or call 101. If you're abroad, report it to the local police straight away and get a written police report or crime reference number before you leave the station. You'll need this for any insurance claim, and some insurers won't pay out without it.

If you have phone insurance (through your network, bank, or a separate policy), call them with the crime reference number to start a claim.