Download and install GeoGebra here: https://www.geogebra.org/download
GeoGebra downloads by Operating System:
On Mac, once downloaded, please unzip the downloaded file, right-click (or ctrl-click, two-finger click on a trackpad) on GeoGebra, and then click Open.
Note from the teachers: "Please make sure that students are installing the full desktop version of GeoGebra. GeoGebra also offers GeoGebra for Tablets (including iOS, Android, and Windows tablets) and Chrome, which is not as fully-featured as we need it to be."