Welcome to CDFreaks!
A nice place to start is PortForward.com. You can find a lot of info there about what is and how to do port forwarding.
> I want to forward a port for utorrent but I don't know if it is safe enough
In my PC I have done the following as safety measures:
a) Disable uPnP from both windows and router
b) Configure Î¼Torrent to use the same static port for incoming connections (not random)
c) Configure my software firewall to allow TCP/UDP incoming connections to that specific port, for utorrent.exe
I think I'm quite safe. Remember, you can never be 100% safe when connected to the internet
This of course doesn't mean that you can't be relatively safe
> how can I be sure that the static ip address i choose isn't
> allready in use?
Are you behind a router? How many PCs are connected to it? If you have a router and only one PC is connected to it, then you don't have to worry: pick any address you like in the form xxx.yyy.zzz.aaa, where xxx.yyy.zzz is the prefix of the IP address you use to connect to your router. Eg, if you use 192.168.1.1 to connect to your router, you can pick 192.168.1.aaa, where 1<aaa<255.