You want to take a look at the PhAB tool. It will allow you to create the look of your GUI. You start with a window and place appropriate “widgets” in the window. Widgets are things like labels, and text strings. There is a special widget for drawing whatever you want. It is even possible to build your own widgets.
Photon programs are for the most event driven. Events are things like mouse clicks, but can include external events. You write pieces of code called “callbacks” which are attached to events. Each widget has a very long list of related events that you can attach to.
This description is way to brief but it might serve as a place to start. There is a tutorial in the docs on doing your first Photon program and I suggest you take a look. If the demands on the program you are writing are large and your internship is long enough and there isn’t local knowledge at your company in this area, I suggest you look into a class.