It's relatively simple. Normally your router blocks all incoming requests from the internet side (WAN), including access to your ip camera. So you need to open a port on the router's firewall and have it forwarded to the port used by your ip camera (probably 80). You can use any port on the firewall for the external port, but it's best to use something else, something above 1024, such as 8080, or 9999, etc.
Of course, now that your ip camera is exposed to the internet, you’ll want to use a strong password to prevent unauthorized use.
Another more secure (but more complex to setup) approach would be to establish a VPN into your home network (e.g., PPTP, OpenVPN), or even use SSH. This would allow you to access ALL of your home network devices as if you were sitting there locally.
But at least for now, port forwarding is the easiest solution.