My screen keeps freezing when i open a app, I cant turn my phone off. what should I do?

You should try to do a restart of your phone. You will not lose any data when doing this. Press and release Volume Up, then press and release Volume Down, then press and Hold the power button till you see the Apple Logo on the screen. Ignore the slider that pops up telling you to swipe to turn off your phone. Just continue to hold the power button, the phone will turn off, and then back on and you'll see the Apple Logo. Once you see the Apple Logo you can release the button and let the phone continue to boot.