Once you have your Pi and SD card, the next step is to install an operating system onto the SD card. Previously this was a bit difficult, especially for beginners, because of varying main operating systems (macOS, Windows, Ubuntu, etc.) and their different methods of accessing the SD card.
It's as simple as choosing the desired operating system, selecting the SD card, and hitting WRITE.
Head here, then download the application for your operating system by clicking the corresponding link.
Raspberry Pi Imager is supported by macOS, Windows and Ubuntu.
Once the software is downloaded, open up the application.
First, choose which operating system you'd like to put on the SD card. Most will go with Raspbian but you have quite a bit of options here. For more on this topic, see Choose your operating system below.
Next, insert your SD card into your computer and select it inside the application.
Power user trick: press Control+Shift+X (Windows) or ⌘+Shift+X (Mac) to bring up the Advanced Options dialog, where you can configure essentials like WiFi before the card is even written, rather than having to change all this on the Pi later.
When you have your software and sd card selected, go ahead and hit "WRITE". This will place the selected OS onto the SD card.
Depending on the size of the operating system you choose, this may take some time (10-15 min)
There are a bewildering number of operating systems that you can install on your Raspberry Pi. If you are a beginner, you'll probably want to stick with one of the Linux distributions.
Having decided you want to install Linux, that is not the end of the story. You now have to decide which distribution or 'distro' of Linux you want to install. Being an Open Source operating system, anyone can take one of the existing distributions an add things to it or configure it in a certain way before packaging it up as another distribution option for anyone to use. This is how the most common Raspberry Pi distribution, 'Raspbian', came into existence. The existing 'Debian' distribution was configured and kitted out with useful things like IDLE (a python-programming language development editor) and Scratch (a learn-to-program gaming system) to make it suitable for the Pi.
All of the Adafruit tutorials (and nearly every other tutorial online) will work with Raspbian, and nearly every Pi out there runs it.
In addition to uploading a number of pre-configured OSes, with Imager, you can also ERASE an SD card.