The Arduino IDE is available for Windows, macOS, and Linux and can be installed on your computer with just a few clicks.
You can find the current version on the official download page at arduino.cc. Clicking on your operating system will start the download.
Once the file is on your computer, run it (or open the ZIP file) and follow the instructions. Afterward, you can open the Arduino IDE.
Alternatives
The Arduino Web Editor is also officially supported. With this, you can program your Arduino directly in your browser. However, this doesn’t work completely without downloads, as you’ll need the Arduino Create Agent plugin for communication between your computer and Arduino. Additionally, you must create a free user account to use the editor.
Experienced users can also use the Beta Version, which you can find further down on the download page. This version already contains future features. However, the testing phase is not yet complete, which means bugs may still be present. Therefore, beginners should avoid using it.