I'll post with the caveat that I don't actually use Trisquel (although I do
use Debian so pitchforks and torches down please).
The Trisquel kernel will have firewall capabilities, configurable through a
program called iptables.
How familiar are you with GNU/Linux in general?
If you're not, and don't have much time to learn, then I guess installing a
graphical firewall configuration tool like ufw might be for you. I'm sure
actual Trisquel users can give you better advice on which are good, how to
configure them, etc.)
If you're experienced or are willing to do a bit of learning, I can recommend
Debian's "Securing Debian" manual.
(http://www.debian.org/doc/manuals/securing-debian-howto/ch-sec-services.en.html#s-firewall-setup).
This also includes information about graphical firewall configuration tools.
As Trisquel is based on Debian I guess most of the information is applicable,
if a little bit out of date by now.
The script included in that manual doesn't take advantage of improvements in
boot sequencing, and has what I believe is a bug in it. I include the script
I use on my personal computers (minus the actual ports I leave open, which
you will have to decide for yourself by adding them to the
REMOTE_TCP_SERVICES variable). Once you've put your firewall script in
/etc/init.d/ just run "insserv myfirewall", where "myfirewall is the name of
the script.
Let me know if you need any clarification or guidance.
