Install a very simple and uncomplicated forewall that is designed for Linux and works perfectly for Ubuntu.
sudo apt-get install ufw gufw
UFW is command line but very simple tool to manage your firewall however you can also use GUFW in gui format or within X Windows for a user friendly graphical interface.
Block too many ssh or brute force attacks
sudo apt-get install denyhosts
Usually it protects your ssh services automatically and can send emails with reports however further configuration can be made in /etc/denyhosts.conf