Semagix

I can programm in Java and would like to programm a peer2peer
application(like Skype).
Where should I start?
Where can I find informations?
At first think about what you want to do. Make a concept for your P2P
network. Should it be Serverbased (like Skype or eDonkey, read
serverbased as “at least depending on privileged nodes”) or “really”
P2P like Freenet (I think it is)?
When you know what kind of network you want to build, you may read
the source code of applications which build a comparable network.
If you want to use Encryption, look for open implementations of well
known an accepted algorythms.
Also look at sourceforge or other opensource sites. Maybe there is
already a project which provides the services you want to create.
Then you may use existing code (respect the licences like GNU GPL
etc.!) or contribute to it.
Try to make your program as portable as possible. It is sad to see
lot of Java projects which execute only under Windows.
You wrote, you want to write a skypelike Application. In skype they
found several things that forbid a use in company networks.
If theres no need to do so, try to write your networkcode in a way it
does not interfere with common security guidelines. Of course
depending on the intended use of the software – a hackertool may
behave stranger than a business application.