Just setting up a new Raspberry Pi sd card so am going through some of the steps below. How
I had my old sd card so needed to delete this first so it can be used again, for this I used gnome-disks:
From here you can see the file system layout, and that the file system is /dev/sdb1.
Highlight the partition you want to remove and click the – icon.
You will need to confirm that you want to delete partitions from the disk,
We can then add a new file system (click the + icon)
I just selected the defaults here, to over write the file system with zeros, takes ages and when you write the Pi image file it will replace anyway, so go with the quick option.
Once all this is done you get something that looks like this
Make a note of where the file system is on the computer /dev/sdb1
Now click on the power icon in the corner this will power down and unmount the drive.