I presume you are doing a custom installation.
This is from memory, so subject to correction: choose sdb and remove the
existing FAT32 partition(s). Choose add, 'use this partition as root' ('/')
and use ext4 as the filesystem. It's a good policy, but not essential, to
also make a home partition. Root can be 10 to 25 GB in size and the rest,
except for a small linux swap partition, is for 'home' which is where all
your personal data is stored. Swap is the same size as your RAM.
I hope this helps.