Create a simple Client - Server application. It can be in Java or the language you fancy the most. If you are new to Client - Server stuff, start with the language you are the most familiar with. Then, if the university you plan to attend use another programming language, try to re-create the same program in a environment that mimic what they use.
I've seen people that weren't so bad at programming, but never programmed a simple client/server application. Or others that weren't familiar with Linux nor Java. The first few months are very hard for these people. There is enough stuff to learn already in the class, learning a new OS and/or language at the same time is usually not cool.
A typical client/server app you could do is the "math provider service". Basically, your server listens on a port and the client sends requests. To keep it simple, you can send plain text request (eg: "add;12;34"), the server can reply something like "answer:46".
It's nothing too complex, but if you never did anything like that, I would really really do that before starting your master.
Get to know "how" to do stuff. It is very important. You can expect the master to be quite a bit theoretical, they probably won't teach you how to open a socket, bind it to a port, what are the kind of exceptions you can expect (yeah, I'm programing in Java) and so on. You will most likely be expected to implement locking algorithm, load balancing stuff and so on. But in order to do those, you need to understand how to do the basic stuff!
If you are a beginner, I recommend Distributed Systems: Concept and Design by George Colouris (the name may be wrong a bit, that's on the top of my head). It covers pretty much everything, the basics, Lamport, TCP/UCP/IP. For instance, the java examples of client / server in that book are, imo, much better than anything you'll find on the net. There're not totally different (a server is a server...) but I think they are much easier to understand.
Once you are familiar with this, you may want to try something more powerful. In my experience, you'll spend a lot of your time configuring stuff (why it isn't working? Ho! It's the claspath! Ho! That file is not world-executable! Ho! These classes should be in WEB-INF/classes, not WEB-APPS). So you can try to setup a little Tomcat/Glassfish server at home. Try to run SOAP or JMS on it and code a client that uses those services. Then, you'll be miles ahead your colleagues :)