These Tutorials are an excellent place to start learning how to use GeoGebra. GeoGebra works not only on desktops but on phones and tablets as well.
The Manual is comprehensive and note the Quick Start tutorials which are very clear. You will also find manuals and much helpful documentation on the same page. The great thing about GeoGebra is that so much has already been written you can probably find what you need already online!
Mr Rowlandson (@Mr_Rowlandson) has written a booklet of training activities for teachers learning to use GeoGebra.
You can also use GeoGebra’s YouTube channel to watch demonstrations.
GeoGeoGebra is not just for Geometry, note how good it is for Statistics too, copy in those large data sets and get analysing! You will find a quick start tutorial for all the main features of GeoGebra:
Quickstart Tutorials
GeoGebra Geometry Quickstart
GeoGebra Algebra and Graphing Quickstart
GeoGebra 3D Geometry Quickstart
GeoGebra Spreadsheet Quickstart
GeoGebra CAS Quickstart
GeoGebra Probability and Statistics Quickstart
As mentioned above much is already written for GeoGebra, for example thinking of older students try MEI’s very helpful advice on the Use of Technology, also on Integrating Technology into schemes of work for older students (UK A Level age16-18). Note that tasks are also given by type of software including GeoGebra.
Another source where you will find GeoGebra used to help students understand and explore Mathematics is Underground Maths where many tasks have associated GeoGebra resources. An Underground Mathematics search on GeoGebra reveals the extent to which it has been made use of!
Note also some excellent examples from the ATM Conference 2017.