For all the information you need to learn how to use this outstanding app for Mathematics see “What is the GeoGebra Graphing calculator?” and all the Tutorials available.
Having downloaded the app to my phone I can confirm it works really well and i was easily able to follow the instructions given in the links here.
I very quickly created the diagram in the screenshot from my phone below. Using a finger one can drag point D around and note the angle at the circumference. With an account you can also save your files which are standard GeoGebra files.
GeoGebra App