The easiest should be to start in "recuperation mode" (the second line for
Trisquel in GRUB, the bootloader). This provides you a root shell what means
you can do whatever you want (including destroying your system). To change
the password for a user named "banana":
# passwd banana
To create a new user named "banana":
# adduser banana
First check the manuals for these commands:
# man passwd
# man adduser
If you create a new user, you may, in particular, be interested in adding
this user to groups that let him administrate the system!
