You'll want to make sure you have all of the required accessories. For this project, you'll need, at a minimum, the Raspberry Pi, a 2.5A power supply, and a Micro SD card. Links to the required accessories are available on this guide in the "Tools" section.
The first thing you'll want to do is flash your SD card with the Raspbian operating system image of your choice. There are a few versions of Raspbian you can choose from and the instructions for each are nearly identical.
I wrote a separate guide on installing Raspbian Buster. Use that guide to install the Raspbian operating system and then return here!
Now, with your pi powered on and connected to your local network, you should be able to ssh into your pi. SSH comes installed and enabled on Raspbian, so you should be able to SSH in without any prior setup.
First we'll ping the Raspberry Pi to make sure we can find it. The default hostname for the Raspberry Pi is unsurprisingly: raspberrypi.
You should see output like this:
PING raspberrypi.attlocal.net (192.168.1.243): 56 data bytes 64 bytes from 192.168.1.243: icmp_seq=0 ttl=64 time=81.131 ms
Then press ctrl+c to stop.
Since we can reach our pi, now we'll SSH in. The default username and password for the Raspberry Pi is pi and raspberry.
ssh -l pi raspberrypi
The -l option specifies the username, which in our case is pi. Then we'll type in the password: raspberry. For security reasons, you won't see the password as you type.
Now you should be logged into your pi, and you should see a prompt like this: