Sounds like your display is faulty, depending on when you purchased the device you may still be under warranty. First step would be to visit apple.com/support and check warranty status. If there’s no other damage to it it may well be covered and they can walk you through the process.
If not you can get it fixed through Apple if you’d like at a cost. It varies between countries a little but visiting https://support.apple.com/iphone/repair/... should give you some idea.
Given that we’re on iFixit, if you’re confident with electronics repair / can follow the guides and want to give it a go. check out https://www.ifixit.com/Store/iPhone/iPho... It sounds like a screen replacement would sort the issue.
Hope this helps!!