Format partition on 2017 iMac HD

First make sure all of the data on the iMac is backed up preferably to external media that can be accessed by any OS.

Second, make sure to create a bootable macOS USB installer so you can easily reinstall macOS if you decide Linux is not for you since Internet Recovery Mode doesn't always work as advertised. Also make sure to test the macOS USB installer by booting it.

When running the Ubuntu installer let the Ubuntu installer select the whole drive to use which will destroy all the macOS partitions. There are a lot of websites and YouTube videos showing how to install Ubuntu. Most of the websites/videos will be showing Ubuntu being installed on a traditional Windows PC, but the steps are the same for macOS (except for how to boot the Ubuntu installer).

This is a Mac forum. You should check on the Ubuntu support site for an answer to this question.