One way to achieve this is to set up a port redirection on your router.
For example if you want to be able to use Remote desktop to your computer from the internet you would need to set up a port forwarding like;
Service Name: RDP [you can call this anything you want, it is just a name for the port table in the router]
Protocol: TCP
Wan IP: [public address here]
Public Port: 3389 [standard port used by RDP, or whatever you have configured it for]
Private IP: [private address here]
Private Port: 3389