If the disk does not have free space (the normal case if it has ever been
used or if it is in a computer that was sold with an OS), you need first to
remove or reduce some existing partition(s).
Then you add the partitions for Trisquel. You need a root partition (that is,
you choose, from the drop-down menu) to mount '/' on it. You can have other
partitions. In particular, you certainly want to have /home separated from /
(so that you can decide to install another system to replace Trisquel and yet
not touch the user data in any way). Every partition can have a different
type of filesystem (Trisquel's default partitioning uses ext4 for the root
partition and XFS for the home partition).
Something else you probably want (especially for a laptop you plan to
hibernate from time to time) is a swap partition. It is an extension of the
main memory (the RAM) onto the disk. You only need to create a small
partition (say of the size of your RAM) for it. It is not "mounted".
