First, you need to know some Java. More than the basics.
Then you need to know some networking stuff, TCP/IP especially.
Then you need to know some TCP/IP programming (the basics, to understand what is under the ORB).
Then you need to understand CORBA. It's not very difficult. Just a buch of new API ( sort of ;-) ).
Install java JDK1.2 or 1.3
Install a CORBA dev kit. Try www.borland.com/visibroker
for a good trial.
I assume you have a win or linux system.
By the way, in my case, it took me 3 months to understand what's all about CORBA and I am still learning (I am not the brightest around).
Best of luck.