To install Ubuntu on your Acer Nitro, first download which version (either 18.04.2 LTS or 18.10, as of writing) you wish to install. I would recommend the LTS version.
I see that you have Xubuntu and Lubuntu tagged as well. You can download Xubuntu here and Lubuntu here.
Get a USB drive that has a capacity at least as large as the downloaded ISO file. Make sure you have nothing on the USB drive, because everything will be lost. Then follow this Lifewire guide on how to burn the downloaded ISO file to your USB drive. This will take some time, so please be patient.
The Lifewire guide recommends you use Rufus to burn the ISO file to the USB drive. I have used it and it works great. You can download it here. If you choose the portable version, you won't worry about having to install it, so that saves time.
Once this is finished, plug the USB into the Acer Nitro, assuming you were not using the Nitro to perform the above tasks. Then start/restart the computer, and boot into the Boot Menu by pressing the F12 key (repeatedly, beginning as soon as or right before the splash screen or logo appears, if there is one), or the ESC and F12 key together. Then use the arrow keys to select the USB and press the ENTER key to boot into it.
It will give you the option to Live Boot the operating system or to install it. Select the option to install and follow the on-screen instructions to install it. You can use this official guide to help.
If you have further questions after reading this answer and the linked guides in their entirety, please ask them in the comments below.
To install Ubuntu on your Acer Nitro, first download which version (either 18.04.2 LTS or 18.10, as of writing) you wish to install. I would recommend the LTS version.
I see that you have Xubuntu and Lubuntu tagged as well. You can download Xubuntu here and Lubuntu here.
Get a USB drive that has a capacity at least as large as the downloaded ISO file. Make sure you have nothing on the USB drive, because everything will be lost. Then follow this Lifewire guide on how to burn the downloaded ISO file to your USB drive. This will take some time, so please be patient.
The Lifewire guide recommends you use Rufus to burn the ISO file to the USB drive. I have used it and it works great. You can download it here. If you choose the portable version, you won't worry about having to install it, so that saves time.
Once this is finished, plug the USB into the Acer Nitro, assuming you were not using the Nitro to perform the above tasks. Then start/restart the computer, and boot into the Boot Menu by pressing the F12 key (repeatedly, beginning as soon as or right before the splash screen or logo appears, if there is one), or the ESC and F12 key together. Then use the arrow keys to select the USB and press the ENTER key to boot into it.
It will give you the option to Live Boot the operating system or to install it. Select the option to install and follow the on-screen instructions to install it. You can use this official guide to help.
If you have further questions after reading this answer and the linked guides in their entirety, please ask them in the comments below.