Now as you can see I use the same port for external and internal port but I don't know why there are 2...?
After that I create the server (with RakNet), with again the same Port. Is this correct so far??
No for the client: Does the client only have to connect (with RakNet) with the external ip of the server and again the same port? Or does he also need to open a port or something?

First, there needs to be two ports, because you need to specify:
1) Which port should your router open to the external world. Say, port 80 for a web server.
2) Which port should it forward to on the inside. This may be different -- you might be running Apache locally on port 8080.

Second, yes, the server needs to listen to the port that you told the router to forward to (not from).

Third, yes, the client should connect to the external IP and port configured. Note that clients within your local network may not be forwarded, depending on whether your router does "hairpin" NAT or not.

Fourth, UPnP is a broken concept. First, it's a bad idea to give random devices on your network control over how the firewall operates. Second, what if two different machines within the network want to host the same game on the same port? The alternative is to use an introducer/match-maker server on the outside of the NAT, which can also do lobbying and game discovery/server lists. That approach has none of the problems of UPnP, and usually works even if the home router does not support UPnP (some don't support it; some come with it default off; some are intentionally disabled by people who don't want random programs re-configuring their firewall.)

So I could use a different internal port but I it's not really necessary I mean it also works with the same port right?

[quote name='hplus0603' timestamp='1314567832' post='4854832']
Second, yes, the server needs to listen to the port that you told the router to forward to (not from).

I don't really understand that could you explain this?

[/quote]

What you need is a good primer on how the Internet Protocol suite works. How does IP work, how does UDP work, how does TCP work, and how are hosts and services addressed on the Internet, and how does routing work.
Without that understanding, it's unlikely that your network code will be robust enough to work on a larger scale.
If you don't want to bother with studying right now, then I suggest you simply manually configure your router's port forwarding or DMZ for now, to make it work for your current case.