Click on your HDD/SSHD/SSD in the sidebar menu and click erase at the top of the window.

Select Mac OS Extended (Journaled) or APFS and name your HDD/SSHD/SSD to whatever name you would like it to have.

The format APFS is required in Mojave to get the usual software system updates. If you prefer to select Mac OS Extended (Journaled), you won't get any system updates.

APFS will work on the End 2009 models and later as with High Sierra. If your device doesn't support originally High Sierra (and therefore isn't able do start with APFS) , you won't have any recovery partitions and the reboot display will be different.

Once you have successfully erased your drive, click the words "disk utility" in the menubar. Click close and Disk Utility should close.