install ssh
First things first. You need ssh installed. Not just the client, the server too. Find out:
$ which ssh
/usr/bin/ssh
$ which sshd
/usr/sbin/sshd
If which can't find them, you need to install: sudo apt-get install ssh sshd.
The install process should set up everything, but just in case, make sure the ssh port (22), is open (if ufw is disabled, consider enabling it): sudo ufw allow 22/tcp
what is your address
Type ip addr | grep inet to get your ip address. If it starts with 192.168., 172.16. up to 172.31., or 10., you have a local (aka private) ip address. Otherwise it is public (eg 74.125.224.51). If it is public, you have it easy. Anyone can try and connect with a command like ssh bob@74.125.224.51.
If you are stuck with a local IP address, you need to configure your router to forward port 22 to your computer. Then you can go to a site like http://whatismyip.org/ to get your public ip address, and anyone who has it can try and connect with a command like ssh bob@74.125.224.51.
get a permanent address
Sign up with http://dyndns.com/ for their free Host Services to link a dyndns name to your ip address. I'm assuming here that you have not bought a static ip address from your ISP.