Okay, don’t follow that tutorial. It’s a separate website that’s just hoping to get clicks. What you need to do is go onto raspberrypi.org, go to help, and select get started. Simply format your SD, download the NOOBS zip file, unzip the file, move the contents of your file to the SD, and boot your Pi. You should be greated with a screen after a few mins asking you for setup options. Select Rasbian, the recommended OS, and wait for the setup to be complete. Next, follow these instructions to setup your RetroPie: https://www.petrockblock.com/2012/07/22/retropie-setup-an-initialization-script-for-retroarch-on-the-raspberry-pi/.
I really hope I’ve been able to point you in the right direction. You should know how to do all this if you’ve owned a Pi in the past. If you forgot, then maybe you should refamiliarize yourself with your Raspberry Pi before you get started on this project. It’s not something that you can do very easily without some basic understanding of your Pi. Good luck!