Force an app to quit
In the app switcher, hold a finger on any of the app thumbnails until red buttons appear at the top left of all of them. Tap one to close the corresponding app. Do this only for problem apps, because you risk losing unsaved data. If an app uses lots of power in the background (find out in Settings > Battery), consider disabling that privilege (General > Background App Refresh).
Set up notifications
To open Notification Center, swipe down from the top edge of the screen, except from the top-right corner (which is reserved for Control Center). If you use the front-facing camera a great deal, though, consider swiping down from the area to the left of the notch, so that you don‘t end up smudging oil from your fingers over the camera area and need to clean it quite so often.