this is actually a tricky question. I would say that this depends of the situation.
When you try to access a computer through a router, there are two cases possible:
1. you have legitimate access to the router (and the computer)
In this case, you can ensure that the server run on the computer you want to access (i.e. if you want to access the web server, you will ensure that the server is on and accessible by the port 80 (for example)).
Then, you will have to connect onto the router to set a port forwarding. This means that you will be able to select a port number (probably using the tab port forwarding) and you will be able to set the port and the address of the machine you want this port forwarded too. This means that when somebody will try to access the service provided, it will connect to the external address using the port you will have first entered. Then, the router will convert the port number to the port number of the internal network, and "pass" the packet onto the internal network to the address you will have specified.
2. you don't have legitimate access to the router
Well, then, the question shouldn't be answered as this would be illegal. One way of doing so would be by scanning the router for open port, finding servers, and trying to "hack" them or one of there specificities (eg sql injection using a web and database server). But this was not your question, I bet
If you have any more question, don't hesitate to contact me personally.