If you are setting up a production mongodb on Ubuntu a certain level of proficiency(or access to some who as) is going to be necessary. Aside from installation and configuration there will be a need for ongoing maintenance of the system.
If you are setting up a system to learn mongodb, xfs is not going to be required.
Mongodb Atlas alleviates this and allows you to concentrate on mongodb.
You are going to need to create or identify a partition to use and format it XFS.
There is no reason not to use XFS for the entire system, RHEL7+ use XFS by default, you’ll have to reinstall though.
Operating on existing partitions and filesystems can be a destructive process and render your system unusable.
You can use disks(graphical) to partition and format.
Or fdisk/gdisk to partition and mkfs to create the filesystem.
The filesystem will need to be mounted at boot for mongod to auto start, disks again can do this otherwise you can edit /etc/fstab.