Put out any flames, taking care for your own safety. Cool the burn for at least 20 minutes with plenty of clean, cool water. Cover the burn with cling film if available. Don’t try to remove anything that’s sticking to the burn.
Always carry a first aid kit – you might never need it, but it could save a life. Learn first aid – you can get first aid training from a qualified organisation such as St John Ambulance, St Andrew’s First Aid, British Red Cross, or any suitable, qualified body.