This is quite a straightforward operation and the principles of port forwarding (or where to find the info) have been pointed out by others, but just a few more tips:
1) Port forwarding is a feature of your 'outside world' connection device - maybe a cable or ADSL (broadband) router so you'll need to check its docs - probably it has a Web interface for configuring this.
2) Every man and their dog knows that the ssh port is number 22, so if this port is open to the outside world, you will get a lot of attention from elsewhere trying to login using user/password tables etc. To help with this, see if your router supports forwarding alternate ports so, for example, you can instruct it to forward (say) external port 40822 to your computer's port 22 and then you would ssh connect to port 40822.
3) In the long run, if you are going to access your computer regularly from the outside world, you should read up about setting up OpenVPN (or another VPN system compatible with your OS) to give you a secure tunnel into your machine.