Hello there!
It is best to consult a specialist who knows what they're doing to see what is wrong with the phone. Based on the looks from one picture, it is hard to tell if it is just screen damage or there is a bigger underlying issue. Again, people at stores like the Apple store and many AT&T technicians know a lot about phones (according to my algebra teacher, who used to work there) so it is best to get professional help, since there are many harmful chemicals under your phone's screen, as well as possible broken glass parts from the screen could get into your fingers, it is not wise to fix the phone yourself unless you are fully aware of what you are doing.
You can also get a refurbished phone online (not sure about in store) From prices around $20-50 from an iphone 4 or $40-90 for an iphone 5, not sure which model is listed in the picture.
I hope this helps and have a great day! Best of luck with your phone :)
