Which ipaddresses/domains do I have to set in my whitelist, to enable skyp? Outgoing traffic to ports 80/443 is restricted to domains in my whitelist. Incoming traffic is not restricted for ports 80/443.
Regarding incoming traffic: your Skype instance will randomly allocate a port to listen to incoming TCP and UDP connections. You can see the port number in "Options, Advanced, Use port XXXXX for incoming connections" and configure your firewall to permit incoming connections.
Regarding outgoing traffic: you shouldn't really block any outgoing TCP or UDP connections if you want direct connections to your peers and resulting high-quality, low latency calls.