I've had my phone for around a week. I charge it overnight (100%), but when I unplug it, 6 hours later, it is at 0%. It is turned off the entire time (including sound). Any suggestions for fixing it/preserving the battery? Thanks a bunch.

Take it back to the shop as it clearly has a fault.

If you have any apps installed on there that modify the phone such as keyboard changers or battery icons or your home screen delete them. Check if bluetooth is turned on and turn it off if it is. It actually takes more battery for it to vibrate than to keep sound on. Hope this help 😀

Usually this happens when people keep all the services on all the time. Turn off Bluetooth, Wifi, Location (GPS) when you are not using them. Do you know how to do that?