QGIS is an Open Source Geographic Information System that currently runs on most Unix platforms (macOS/OS X included) and Windows. Developed using the Qt toolkit (https://www.qt.io), it's mainly written in C++ and Python languages. This means that QGIS feels snappy to use and has a pleasing, easy to use graphical user interface.
QGIS source code is available at https://github.com/qgis/QGIS. If you wish to help on bug fixing or adding new features to the application, you are most welcome and can freely contribute via pull requests. You can also get in touch with the other developers through their mailing list at https://lists.osgeo.org/mailman/listinfo/qgis-developer.
Beside this, we've written a short QGIS Developers Guide to help you in QGIS coding. It provides rules, tools and steps to easily and efficiently contribute to QGIS code and is available at https://docs.qgis.org. You are warmly encouraged to read it.
QGIS has a plugin infrastructure. You can add a lot of new functionality by writing your own plugins. These plugins can either be written in C++ or in Python.
If you want to contribute to QGIS Project with your plugin, it's highly recommended to read the QGIS Developers Guide at our documentation website: https://docs.qgis.org
Go to the QGIS repository in GitHub at https://github.com/qgis/QGIS/
Make a Fork for yourself. Learn more about forking and keeping your repo in sync with the main development repo at http://help.github.com/fork-a-repo/
Clone your personal copy of the repository to your system, and add upstream as mentioned in above provided link
Read the https://github.com/qgis/QGIS/blob/master/INSTALL.md about setting up the installation environment and make a compilation with ccache enabled.
Inside the QGIS folder:
Now follow the instructions in the screen wizard to create a folder for your plugin. After the required input is given, a folder for the plugin is created. Read the README inside the plugin folder to create a plugin.
Building QGIS now will also build your plugin. After starting QGIS enable the plugin to see it and show its dialog.
QGIS has a lot to offer to python developers too.
QGIS has python bindings so you can automate tasks in QGIS via python. We provide a documentation for QGIS API at https://qgis.org/pyqgis/
You can also have a look into the Python Cookbook in our Documentation site: https://docs.qgis.org/
Particularly interested in python plugin development, you can find some instructions at https://docs.qgis.org/latest/en/docs/pyqgis_developer_cookbook/plugins/index.html
Looking for examples of python plugins, see https://plugins.qgis.org