For starters you should install openssh and use key authentication instead of username and password. Then you need to setup port forwarding through your router and forward port 22 (or whichever you choose to use, 22 is default) to ip that you set for router! IE: port 22 is forwarded to 192.168.0.103. Now let me explain, that address 192.168.0.103 is whats called a private ipaddress. You have a public ip address that the rest of the world sees. So if you go to your router status you should see your public ip which will be something completely different from your private.
private = 192.168.0.103
public = 65.345.87.32 (or whatever yours is)
Once you have ssh and port forwarding setup you will be able to reach your server from anywhere in the world by using the ssh command:
or like this
NOTE: I suggest key authentication if you only intend on connecting from say your laptop that you travel with. If you want to connect from ANY computer you use then you will use username/password which is highly LESS secure than key authentication!!